I don't see the big deal... It just layers his face on top of yours, and Bob Marley was indeed black... That's not even black facing. Black facing is when you apply large amounts of exaggerated black makeup to make a white person look black for a role... Layering a black person's face on top of a white person is not black facing. If you wanna be annoyed that they did it on 4/20, sure. But, I don't see how you say this is racially offensive.
And before anyone comes at me to try to say I'm white, so of course I have no problem with it (because that's the generic counter-argument on ATRL), I'm black.