Inspiration comes from many, many sources. If you go looking for these things, you'll find hundreds. Most of the people who give answers here, probably read about them in a book in the first place. Many of the others are simply made up on the spot. In both cases; an extremist looking for them will find the same answer.
Even if there's the miracle case of someone having a flash of inspiration that no person has heard before, if it's that good it'll spread to both sides quickly and by posting it at least it's out in the open. Imagine if an extremist had the flash of inspiration and nobody had posted it openly before, then there'd be no preperation for it.
Besides; as they say: ideas are a dime a dozen. All of these answers are just pointers. With or without them, it takes a huge amount of effort and planning to actually do anything. Knowing that you can spread a biological agent through the water supply (seriously, how many B-movies have this plot, again?) is one thing, actually getting your hands on the biological agent and access to the water supply is a completely different affair.
Now, if people were starting to give real-world addresses on where you can buy said biological agents and names of people working at water treatment facilities... but that'd be illegal, so we don't need any extra rules against it.