If the rules indicate that, then it's fine. Here's a bounty with a similar rule and I hope some other bounty manager can adapt such rule into the campaigns they will manage in the future.
https://bitcointalksearch.org/topic/bountyendeddecentralized-lending-platform-and-mobile-banking-app-5182388 campaign is managed by Arteezy
Yeah, exactly. If the rules state right from the beginning what will happen with your stakes when you leave campaing early I am totally fine with it. Even when you then lose your stakes if you remove your signature. But then they should also provide a fixed date when the bounty will end. This way I can decide before joining if I wanna keep the signature that long. And if they extend the campaign afterwards everyone should be free to remove the signature/leave the campaign without any penalty.