It's very easy to message "Free for OSS, paid for other folks", and it prevents you from having a failure to convert moment when a) you start using Github internally at a job/startup/etc and b) you need to add the second repo. If you then have to go get authorization for a purchase, you might do something stupid like a) migrate or b) cheat, rather than go through the authorization process, because you've got work to do today.

It is much easier to say "Look, you know Github, you like Github. You want to use Github for work? Get them to fork out for it. No, not fork(1). fork($)"

Hypothetically suppose a particular company's advantage is UI/UX. If this is true, that company is in less than zero danger of getting FOSSed.