Hopefully some aviation attorneys will be interviewed on this topic. I believe the fact that US airlines are considered "common carriers" also influences their ability to refuse service. The United CEO has already changed his mind about how to message this. He said this on Good Morning America today:

"We are not going to put a law enforcement official to take them off...to remove a booked, paid, seated passenger...we can't do that."

That's hard to reel back in later.