I would chalk this up to bureaucracy rather than any attempt on U.S. musicians' part to exclude UK folkies. It's not like U.S. folkies have any clout when it comes to who gets visas and who doesn't anyway. I also wouldn't attribute it to the fact that they were FOLKIES, per se. The point being that they weren't likely to be a big-money draw, which covers a lot of other occupations/attractions in addition to folk music.

And please don't try to make sense of U.S. immigration. It doesn't make sense to most Americans, either.