R rating was most likely for the language, which did get salty in a few places and also there are a few bits of fairly gruesome gore (Ving Rhames’ half eaten corpse for one). I can also getting an R for the tone, since I have heard of the MPAA slapping harsher ratings on movies for such things. It’s kinda campy and silly, yea, but it’s really dark in several places.
You are right that both Mother and Father are from Twin Peaks and they did play a couple on that show, as well. I think Craven wanted them based on those characters.
They are supposedly going to do a PUTS 2 that is going to be an absolute abortion of a film. I have no idea how they got A.J. Langer to commit to that garbage. My guess is she is broke.