A lot of women, including even nudists, have a complicated relationship with their pussies. When they see a "spreader", they project their own insecurities about being completely exposed. "How can she just lay like that?" is really about how self-conscious they would feel if exposed in similar fashion themselves.
My wife has herself admitted that she's really enjoyed sunbathing (privately) with her legs dangling apart. So it's not that she's against spreading; it's just that she feels that she would be making a "spectacle of herself" if she did it continuously in front of other people, and especially while interacting with them. Which is why she feels our friend is "doing too much."

At least for my wife, there's also a tinge of jealousy involved. She's uninhibited enough; but when she sees spreaders, she feels that she's actually not THAT uninhibited.

On our third or fourth nudist trip, my wife reconciled herself to the fact that if she was going to be physically active at the resort, SOME exposure will just naturally occur. And she's fine with moments where her pussy is visible to others; as long as it's just "moments."
I would think that it would make nudism very boring and very stale to be so preoccupied with the pussy being visible that you're constantly aware of keeping your knees touching. At that point, just put on a bikini bottom and go to the regular beach...