"They are mature adults, and as such are responsibile for what they choose to expose themselves to."

A sign of a healthy concert series or venue is that the audience will come out and take a chance on artists they are not familiar with. IF the series or venue has a good history, the patron will have an idea of what to expect.   However, the person purchasing the ticket has the ultimate responsibility to know what they are seeing.

In the U.S. at least, I think anyone who goes to a concert that is labeled as "folk" should have some expectation that the possibility exists of political content being raised.