Agreed. If this were just some regular frat boy at a Halloween party, then I probably wouldn't give a hoot. I've seen worse.
But he's a royal. That comes with a bunch of privilege, and with it an expectation that he'll at least act the part.
It's a matter of resonsibility. If he can't see why people would be offended then he is dumber than I thought. But I never thought he was that smart to begin with.