Radio City Music Hall is a private venue, they can ban or exclude anyone they want, as long as it's not due to membership in a protected class. You don't have a right to attend a Rockettes show, and if she doesn't like this policy she can start her own Christmas Spectacular show.
I don’t think anyone is arguing that this is outside of their legal rights.
Things can be undesirable without being illegal, and just as MSG can legally punish this lawyer for distant connection to a lawsuit against them without violating the law, other people can note the behavior, criticize it, and even punish MSG for it in their business relations without violating the law.