From their perspective, they weren't doing anything that abused their privileged position. Tabnine trained their model on open source code, much of which was probably hosted on GitHub. Why should GitHub have to ask permission if tabnine didn't?

Whether training an ML model on code is fair use is still an open question, but I don't think GitHub is a greater villain here than anyone else doing the same thing (at least until they start using private repos).

Why would they need to ask for opt-in if they believe what they're doing is within the license terms?

I don't ask for opt-in when I copy a snippet from Stack Overflow or use some code from an MIT licensed repo.

The only question, I think, is if what Copilot does is actually in compliance with the licenses.