AFAIK, Walmart still allows overnight parking, however, in more and more areas the municipality they are located in are passing laws to prohibit it.
This past winter the DW & I were visiting family in Naples Florida, we usually get in late, spend the night in the Walmart, then go visit them first thing the next morning. This year we noticed 'no overnight parking' signs. The signs clearly stated this prohibition was based on a City ordinance.
Since we arrived late and were both beat we decided to stay anyways. Even before we'd finished 'dewatering' the dog a Sherriffs Deputy stopped by, before we could say anything but 'hello' he said 'oh, you're from Canada' after spotted the plates on the m/h. He quickly pointed out that overnight parking was prohibited, but then said 'but don't worry about that, it's just to prevent long term campers'.
Apparently the economic downturn created some problems for municipalities and store / plaza owners.