Sounds like you kinda figured it out. The idea is to stop someone from putting everyone else out of business and then raising prices. WalMart doesn't sell goods below cost -- at least not largely and not as far as I know. Actually other retailers are much more notorious for running loss leaders on certain items whereas WalMart is known for regular low prices. WalMart beats everyone by being bigger than everyone else, which is a different problem than price fixing.