I found only one instance where this shouldn't be allowed:
- If you wanna do this twice (to set a bounty again on the same question).
Also, from this answer (by 40 votes):
"Abuse potential aside, it gives users an incentive to delete perfectly good questions."
But I think this is not really a problem because you can delete the good question (had a bounty), so you can get your points back and then repost again the good question but this time, to leave it as it is.
Also from this question (mod quid's comment):
"the point why it is not refunded is that somebody could for example put a bounty on an essentially unanswerable question all the time if it were different. The main point is added visibility. For this to stay effective they must be limited. So they "cost" something no matter what."
mod quid says: they must be limited and yes, I agree with mod quid's piece of comment. But I also think, what's the difference between one or another question? I did already 'set/paid an space' for my question with bounty, it didn't work. Now I'd like my points back because I have another question.
I think one should be permitted because what if one is limited on points and have different questions (from different courses). So one may want to now spend their points on a new question.
And it does not seems fair to loose your points just because no one answered the bounty question.