It's not so much a problem if the band's audience is an echo chamber and the band can reinforce bias. But if the audience is a mixed bag then political pontificating from the stage is like spitting in the face of paying customers who might disagree with what they have to say. You didn't hear Mick Jagger banging the drum for Thatcherism at Stones' concerts because Mick Jagger was a big admirer of Margaret Thatcher in the 1980s.