I suspect that the "purist" bugaboo first appeared defensively and self-righteously in the liner notes accompanying self-described "folk" groups of ca1960 who were clearly "pop" (or at least "semi-pop") groups.

If they were responding to anything but their own consciences, it may have been to perfectly appropriate criticism by academics that their lyrics, arrangements, and performances seriously misrepresented tradition even while their publicists proclaimed to the masses that they were delivering "the real McCoy."

I can think of a few names but won't mention any.