Politeness

Yesterday at The Coffee Shop, there was this guy in there who clearly is cognitively impaired in some way. Talking to a guy from Africa and his friends about how in 200 years all the black people living in the U.S. will turn white. "It's all about vitamin D absorption, you see?" And on and on he went.
Finally I heard the polite man from Africa say "Sir, I do not like what it is you are saying to me right now and I would like you to go away."
And he did.
Afterwards the man from Africa says to me: "He's not right in the head. He used to be a dentist but he has told me before there is something wrong in his brain now. He doesn't mean to be like that. I just didn't like it anyway."
Later I heard the same rambling guy talking about his 300 mile hike on the Appalachian trail. Some poor "polite" person who didn't know the line "Sir, I do not like what it is you are saying to me right now and I would like you to go away." sat there and listened for a very long time.
I felt sorry for both of them.