GitHub probably does this to protect itself from potential copyright claims by the copyright owner. In most cases, copyright owners will want Github to stop distributing copies of their code to people whose access they revoked. They won't care whether the copies are in a fork or not. If Github didn't revoke access to the code, including in forked repos, then the copyright owners could send DMCA takedown notices and potentially sue Github. Processing those takes time and money, so they do the easier and cheaper thing.