Doesn't it depend on the balance of acts, rather than just 1.

If my money is paying for it, I choose the integrity of the event. That includes the integrity of the organisers. If Chris Wade is still running it she is very definitely a folkie, but a commercial realist. It still begs the question of what the event is called. Put the word folk in the title and there are expectations. Put the word "music" (or heritage) instead and we would know where it is heading.
If the pre-festival concert is a separately paid-for event then I would have no quarms about attending, and not paying for not folk. I do it with Towersey, and Sidmouth.
Beatles ain't folk they is commercial. Bootleg Beatles is copy commercial.