Without this feature, I can push commits up to my repo and claim they were written by you; with this feature, people can tell whether you signed off on those commits.
So what? That implies that someone should believe someone when they say "look, the email address on this commit says this person's name -- it must be that person!". This all comes down to repo ownership. In your fork of a repo, you have the ability to fake any history you want. Why should that bother me? GitHub should stop auto-associating emails to usernames -- that's the actual problem here.