You can fork it, but that's because they published the code under a license that allowed you to do that. I think you'll find the copyright is still theirs. Legally, they own their work. Contract law says that if they offer it under a license that's non-revokable, they can't take that away from you. But they could relicense it so future changes come under different terms. They can even multi license it, so some people get one set of terms. The open source ones. and others get a more set of terms suitable for making money off it when being used by others (Assuming all contributors agree, if it's a multi contributor project, though a single contributor could do that with just their patches but that'd be a pain).

It's the reason, in projects where the contributors don't sign over their copyrights over to the maintainer, that you have to get everyone's agreement to switch license (or remove code that was contributed by someone who didn't agree). Because each contributor owns their code and gets to choose the license it's distributed under.