It's All in the Writing
So, I think the basic answer to your underlying problem is simply a matter of you writing a query whose form is not a good fit for Worldbuilding. My plan here is to pick out a couple issues I noticed in your Meta query and address those and then discuss the difference between wide open or ordinary brainstorming and how we do brainstorming here in Worldbuilding. Lastly, I'll try to help give some ideas for how you can turn your ordinary brainstorming query into one that fits WB.SE expectations a little better! Maybe even get your question reopened again!
The intent of the question being to gather ideas
Obviously. Compulsory Notice: SE is not an idea gathering service. SE is, literally, a forum where querents ask a focused question and get the correct answer to their question. "How can I get XYZ output in ABC app in PRS format?" --- "You do it by 123ing the ABC". Q&A. That's the SE model. Worldbuilding is different, by proud self declaration in that we don't deal strictly in factual things. Here, our questions tend to want XYZ output where ABC and PRS don't exist, don't function or and nowhere near magical enough! Even so, we aren't a free-for-all idea generating service. We do have to adhere, as best we can, to the underlying model and Rules that SE has established for us. Your question was closed as opinion based, in my opinion, because you wrote your question as an ordinary brainstorming question rather than as a WB.SE brainstorming question.
Still it is "brainstorming," and to be frank I have no clue why this is even something against the rules. A good 80 to 90 percent of the questions on this Stack Exchange are "brainstorming" in some form or another.
You have just discovered the fact that WB is a little different from the ordinary SE forums. And the truth is this: all of our queries are, in fact, "brainstorming" in some form or other. I'd argue that you're experiencing this sense of disjoint, this sense of "why am I being singled out", because, again, you wrote your question wrong.
Even a moderator of this Stack Exchange (who later closed my question, but I'll get to that) has some "brainstorming questions: Here Here, especially how it ends with "how?" Here And more.
Don't compare yourself with others! You make yourself look like a high school footballer whining about why he isn't being paid fifty million to throw a ball around. (But I'll get to that!)
If this Stack Exchange is often used to brainstorm reasons to justify something in worldbuilding, why should it even be a rule in the first place? What is the intended purpose of this SE? Yes, I have read the documentation, but I still do not get the point against brainstorming.
Now we've gotten to the point where you will learn why this forum works the way it does. The simple fact is that brainstorming is not against the rules here. Nor are opinions. What is against the rules here is how you, the querent, go about your brainstorming! I can see that you've got an understanding of the contradiction that is WB.SE --- a Q&A forum that invites and uses brainstorming as its basic tool, yet still still operates within the single strictest Q&A framework on the Net!
The real question you should be asking is not why brainstorming is against the rules, because it's not, but rather, how can I brainstorm within the rules of WB.SE? In other words, how can I write my question in such a way that it invites brainstorming yet does not break the SE ruleset as implemented in this forum?
Since you compared yourself to one of the mods, I'm just going to point out that L. Dutch's question was not closed for being opinion based or open ended or for "brainstorming" simply because L. Dutch knows know to write a question (reasonably well)!
If we look at the third query you linked to (parasitic wasps targeting essentially humans), we see that the OP gave us three out of the four classic parts of a good WB question:
- Introduction: A bit of exposition, a broad net to get our attention, a description that focuses us on the specific problem. Here, L. Dutch talks about the wonderfully gross concept of an insect that lays eggs inside a living incubator from which the adults emerge in comically disgusting Space Balls diner scene fashion.
- Description of Knowns: list of key points of facts that the OP already knows, yet has a question about. The OP here lists several pertinent aspects of the fictional world or a specific part of that world he's asking about.
- Question: What it says on the box. The marquee query and the more descriptive query in the body of the question match up and are focused in nature. The problem I am having is finding a suitable target tissue. I bolded this, because this is where L. Dutch went right and you went wrong!
- Criteria: The fourth part of a good query is, I think, viewed by many as optional. It's really not. This section is where the OP should have laid out the criteria by which he would judge any answer as being good or fitting. It helps us give him useful answers; it helps helps him brainstorm ideas that will fit within the framework of his fictional world.
Ah! I said the word brainstorm! As you can see from the six answers, we got essentially five radically different tissue types for the bug to target. That was successful brainstorming only because of how the OP worded the question!
Now let's look at yours!
Right off the bat, you're asking a "why" question. This is a problem: why questions seek responses that address cause, reason, or purpose all of which essentially boil down to motives or thought processes or choices of individuals or corporates and are thus "story based questions". I would have chosen that rationale to close your question.
A point in your favour is that you give us a solid if succinct introduction, that is, your premise. I find your premise interesting, and that's largely why I'm helping you here in Meta --- if you edit your brainstorming question so that it fits our requirements, I would love to be the first one to vote to reopen it!
I can see that you made an attempt to give us some knowns, but I'm not really sure how they fit with your question. I mean, it's interesting, highly entertaining and possibly noteworthy that an insufficiently trained courier might die on the job, I'm just not sure where you're going with that, or how I might want to work that into an answer that would be both entertaining and useful!
You do expand upon your query a bit, but it's still essentially a why question and thus inviting of any and all opinions. While I have never been opposed to opinion based queries, there's a difference between I want to do this, that and the other given the following constraints and what colour should my faction's symbol be. Can you see how we nuance "opinion" and "brainstorming" into different levels of saturation?
And of course, you did nothing to reign in our brainstorming and our opinionising by giving us any kind of guidelines. So, essentially, I could write Because the magical couriers are all hot girls with brilliant violet eyes and prominent boobs; while the muggle couriers are not as an answer and it would satisfy your question as well as any other.
What to do about it!?
Now that you know that brainstorming is not only not against the rules but encouraged, I think all you really need to do is edit your question. I'd suggest not asking a why question and perhaps narrowing the focus to some aspect of the customer's business, for example, on how the magical courier might benefit their business. Second, I think it would be helpful to offer some description of what you mean by "magical" in this context, some description of the nature of the world itself, perhaps some description of how the magical courier works. And lastly, give us respondents some sense of purpose! Give us a sense of what your actual worldbuilding problem is and also what criteria you're looking for in an answer. Like, "a good answer would touch on the following customer needs."