I've slept in dozens of Wal-Mart parking lots driving between NY and CA (8 times now, I think, along several different routes), and on a couple other trips (including a two-week trip in my dad's RV).
I didn't realize there was a friendly community, but I could definitely tell that some people were staying for more than just the night. You'll be left alone - by people from the store and from other travelers - if you want. I am pretty sure I've been to the one in Flagstaff where the photos were taken, but I don't think I stayed there - LA to Albuquerque is a single day of travel for me.
That one in Albuquerque always has a lot of people in the parking lot, but never seemed very friendly. It may be that I was misinterpreting, though... I guess in retrospect the security van driving around all night with its lights flashing may have been at least partially for our benefit and not to scare us away.
The only one that wasn't friendly to this was in Grand Junction, CO. There are signs all throughout the lot saying you can't stay overnight. Yet, I've stayed there many times - including in my dad's RV (it's a small one). I suspect that unless the store gets in trouble with local authorities, they're not going to go out of their way to enforce local laws like that... unless the managers and security people are assholes, which is more than possible I suppose.
Anyway... the last round-trip I did, I decided to go Wal-Mart free. I haven't actually shopped there in years - except when doing these sorts of trips. And what ends up happening is that you buy more than you really need - the little extra space left in my small car always ended up filled with snacks and things I didn't need.
So I mapped out all of the highway rest stops along the route, and stayed in those. Some have signs saying no overnight stays, or two-hour limit, etc... but I've never had an issue. I suspect that what they really don't want is people setting up tents and things like that. Otherwise, if you're just sleeping in your car you're not really bothering anyone.
If the lower route seems odd, it's not direct - I took a detour on the return trip so I could check Arkansas, Tennessee, and Kentucky off the list of states I've been to.