I have a theory about this phenomenon:
It is possible that some thots feel a little bit "less thot" if they keep something to themselves, i.e. not showing nipples or certain body parts like their vagina. It is a strange concept and a twisted sense of logic, but perhaps some ladies believe they are retaining an ounce of dignity by keeping something away from the public world. For instance, some pornstars (certainly not many these days) won't do anal on camera because they feel the need to keep something private.
I know when I am on camera, I never, ever show my pussy, but for different reasons.
lol