If it's part of their strategy then you can't anything about it
If this is what they are saying in their terms and conditions before you sign up then it's up to you if you want to proceed with such a bounty or find another one that does not have a lock up period.
Arrangement like this will be for bounty hunters to accept during the first sign up, if the team give this rules and participants agrees there's no
problem that will discussed, but for sure things is different when after finishing the job and the rules was change due to some issue or developers
decide that's it is requires the hunters doing so, better to have a much clearer rules so issue will not be raised after.