I'm not offended so much as I am frustrated. You appear to be throwing stones while living in a very large glass house.
Why should my preferences be relevant to your parents, or to anyone?
I'm not saying there are no moral absolutes, because there very much are, but they have to be based on something more concrete than what you or I simply feel is right.
I know exactly what the situation is, and for the third time: I am not looking for "some other reason". I completely and 100% accept your supposition that they didn't want to visit with him specifically because he was black.
Why "you just don't do that"? There has to be some logical reason why.
Let me put it to you in a parable:
Snowflake comes up to COTO and says, "You know, COTO, my parents are immoral. They won't listen to country music."
"Why is that immoral?" COTO asks.
"Well... because," Snowflake replies, "It's country music."
"OK," says COTO.
"So obviously it's immoral if they refuse to listen to it," says Snowflake.
"But you yourself don't listen to heavy metal music. Or jazz music."
"Sure. But that's completely different."
"Why is it different?"
"Because we're not talking about heavy metal, or jazz. We're talking about country music. Good, wholesome, American country music. Absolutely nothing wrong with it. Refusing to listen to it is just a sin. You just don't do that!"
"Why?" COTO wants to know. "There must be some principle, or moral law, or ethical axiom, or something on which you're basing your judgment that refusing to listen to country music is wrong, but refusing to listen to metal or jazz is A-OK. What is that reason? 'You just don't do that' is not a reason. There is nothing logical or defensible about 'You just don't do that'. Do you even have a reason?"
Snowflake considers this, realizes at last what COTO is asking him, and explains his reasoning to COTO, "Oh, well, the difference is..."