It's true, it's not fair for those who work for weeks. But these rules will still apply so that all participants can better support the project to completion. In my opinion, there are a number of campaign managers who give participants to their signature campaigns the opportunity to remove their signatures but must be reported in advance. Without reporting, all bets will also be deleted. I think this is fair enough for participants.
why you said it's not fair? as you can see usually that rules already there before you join it
so it will be good if you not join it at first and just choose another bounty
No this is unfair.
Most of the bounty programs won't mention this rule in their thread. But when users want to leave the campaign in the middle, they will impose this rule. You can't permanently stick to a campaign which won't end after a certain period of time. I have seen some campaigns which are running for almost a year. Do you still think it's a good idea to stick into these type of campaigns?