It's a github 'social' issue , not really a git issue. It might also be a legal issue (identity theft). And i'm a bit surprised github let people impersonate others through their 'social' features.
It is really not possible for github to track the origin of commits. For example, consider a fork. If you pull some commits from the original and then push them to your fork then that would look just like this. The only thing I can think of is that it may be possible to track commits if everyone would sign them as they were created, but that would require all users to change, so I don't see how that can happen.
It would be trivial for them to shell out to `git verify-commit ` in order to verify that the claimed originator has signed his commits with a key tracked by github (by email address, which can belong to only one github user).