I bet Wal-Mart has a lot more turnover from employees who took the gig because it was what they could get at that moment, and then they moved on when a less shitty employer comes available.
Maybe a lot of guys "know what they're getting into" but if Wal-Mart gave a shit, those same employees who would potentially develop into decent workers would stay and Wal-Mart wouldn't be a lousy place to shop.
Admittedly there are good Wal-Marts still, and those places are usually a lot more packed with customers.