When people politicize cultural events, booing would just expose one to the mob; that’s why it’s done. It’s safe for the liberal establishment to inject politics where it doesn’t belong, because their view is the majority view in these arenas.
But it is bad manners to do this at an event that should bring people together, but instead alienates those who hold minority views.
Personally, when “the last DJ,” Jim Ladd at Los Angeles’ KLOS does this, I simply turn off the radio for at least half an hour, and either switch to another rock station or pop a CD into the player.