I believe there should be a way to edit the optional bounty text.
It could happen that you simply forget to add important information about the bounty or you make a typo, etc.
I agree! And would like help editing my bounty text from this post. I typed $\pi/4$ but I obviously meant $\pi^4$. I am not sure why I need to bother a mod with this. I can see the other side of the issue: We don't want bounty hunters to have to deal with moving targets where the OP changes the objectives but it could be something like: OP can make changes and a few other users need to sign off on the change.
Anyway: It does happen but it's going to be hard to produce tons of evidence because bounty text is temporary but I suppose we could look at how often mods are bothered with this to try and generate more evidence that this is a worthwhile feature change. If it only happens once in a blue moon (As is suggested in the comments) I would think that it's not worth the change.