Question is still there, so it can be voted on.
Downvotes and close votes are tangential to each other. Close votes are about "Should this question really be here? Is it answerable?" and votes are about "Is it well written? Was it researched before posting? Is there a chance it'll be useful for other users?".
Some questions are well written and interesting, and earn upvotes, and still end up put on hold as unclear what you're asking - if it's not quite clear. There is a chance for it to be reopened if the issue is solved, so why wouldn't people vote on it in the meantime?
The same is with the closed questions that are bad. If it's unclear what someone is asking, or too broad, it still can get reopened later. This does not make it magically any more useful, or show any research effort.
And just by the way, if question is closed and should not be here, why it is allowed to up vote it?
Edit: Sensible alternative would be to allow OP to click something like I will edit and fix it - and hide the question until it is edited, or lock votes (up, down and reopen ones) and show a banner like "This question is being improved right now, please come back later." - of course even if not edited, this banner should go away once question moves from [on hold] to [closed].