Except that I've slept at WalMarts many times since 9/11. As recently as a year ago and all the RV Web sites consider this a current practice and as recently as four months ago a WalMart spokesperson is quoted defending the policy against criticisms of the policy.
There are definitely specific WalMarts where it is not allowed (less than 10%). I pointed out above where you could get a guide that listed all of them (as of mid-2005). But if someone has had no luck in parking at WalMarts it is through bad luck in the specific stores, not because WalMart doesn't generally allow it. From the best of my knowledge as a fringe watcher of the RV community sites, there has never been so much as a state-wide ban on WalMart parking (let alone a chain wide one). If there had it would have been as big news for those communities as if Disney announced the closure of DCA would be for all of us.
Here's another article on the practice and the conflict between those who like it with those who live near WalMarts that don't.