Biden is shameless, so he doesn't care. The man left onstage was snubbed and insulted, and had every right to be angry. This country is embarrassed every day that we are represented by that freak. It's gone far beyond a joke now, as he destroys the country, alienates our allies, and supports our enemies.
He didn't mean to be rude. It's just that he had no idea where he was, what he was doing, and where his handlers wanted him to go. So he simply walked away with a stupid look on his face.