For Chrissakes. "Freedom of Speech" is only a guarantee of government non-interference. It does not apply to a paid performer in a private venue.

People pay admissions to be entertained. When the level of political preaching interferes with that entertainment, the performer is cheating the audience---as has happened in this case, and in the case of Seeger concerts and Guthrie concerts I have attended. The solution is simple---vote with your feet. An entertainer who doesn't entertain is entitled to absolutely nothing, however well he or she sings.