They walked out because he wouldn't say, in private, that black people's lives mattered. It was simply outrageous to see that in this age. Do you really think it is political to say that a human's life is important, in private?

it's possible to believe black lives matter and be hesitant to cave to the demands of a mob.

https://www.youtube.com/watch?v=3iV8X8ubGCc -- as comically illustrated by Seinfeld