This can be a problem, but it's sometimes difficult to ascertain the motive behind a person putting a bonus on an unresolved question. I do not refer to the statement you quoted (which clearly violates the site's requirements). I refer to the suspicion that the bonus points were put on explicitly to secure a week of garnering "off-topic answers". Further, due to the attention brought to the question by offering a bonus, some answers may then be given that expose that error and show why such an idea should not be encouraged, and why it is irrelevant to answering the question. It could become a double-edged sword!
In other words, allowing the bounty to stand so that the question can remain (instead of being removed for something 'off-topic') could serve a useful purpose, though not from the point of view of the one giving the bonus!
However, I do agree with you, that there may be good reason to remove a question with a bonus attached, if there is evidence of a move to accumulate inappropriate answers. I would include in that category the site garnering batches of similar questions in a short space of time. There may be several users involved in that - not necessarily the one user blitzing the site with similar questions, especially if previous ones get removed. If there seems to be a group of people homing-in on the site, to 'promote' a particular religious stance, then we know the Moderators will be alert to this. They will take time and care, of course, and strive not to have any bias.
I came across that sort of problem recently, starting with a question posted on November 7th, 2022, "Can you help me solve this theological puzzle over John 1:14?" I answered it. But two days later the same OP asked a strikingly similar question, even using exactly the same table to make the same point about that claim (that that verse contradicted another verse in that chapter). But, by then, a bonus had been put on the first question. All this was going on during a spate of suspiciously similar questions that seemed to be out to promote a particular religious interpretation. Now, I could be mistaken, and nothing untoward might have been going on with not just that OP, but others who held a similar view. The best course would be to flag such things for Moderator attention, as stated in a comment here:
"I agree you should be able to vote to close questions with bounties on them. You can upvote or voice your support for that on this Meta.SE post: meta.stackexchange.com/q/121448/256282 –
I'm off to do just that, right now!