Not arbitrarily. The prior licensing arrangement between Apple and Microsoft is central to that case.

And it seems that Xerox didn't actually think they gave Apple this permission

Xerox sued Apple, yes--but as even the Wikipedia article points out, it had much more to do with benefiting from the precedent that the case might have established than with anything they thought before. It's more evident--by the fact that they waited so long--that they didn't think they could deny permission (i.e. that 'look and feel' was subject to copyright, as Apple believed)--which ultimately is what the court effectively decided.