What I want to know is: where the hell did these two old biddies come from? And, is there any reason to suspect that they'd ever return for a less political evening of "real" folk music, and therefore be worth placating?

Political differences are one thing, but to characterize balladry in the tradition of Phil Ochs as "hateful" is a bit extreme. And, certainly, anyone should be ready to hear some kind of left-ish sentiment from any American act characterized as "folk." (Whatever the merits, or lack thereof, of using the phrase "knee-jerk liberals," the more likely political orientation of the typical US folkie is not a well-kept secret.)

That said, I would probably have given them their damn refund (or half, if I were thinking fast enough), just to hush them up and get them the hell out of the house as quickly as possible.