For the most part, I see people parking, slides in, in the back 40 of the WM lot. People do follow the rules and try to not abuse the privilege (and yes it is a true privilege that WM allows RV-ers to stay on their property.)
The main reason I see WMs disallowing overnighting access are local municipalities and their ordinances. Either they think all RV-ers, even those needing a nap must go to a CG, or that everyone with a motorhome is a potential threat or criminal.
I have not seen anyone with slides out, awning out, camp chairs under it, grilling wieners. But I'm sure eventually, I'll come across that.