As they say, TANSTAAFL - There Ain't No Such Thing As A Free Lunch. Department stores can't offer bikes at those prices and still make money if they're the same quality as a quality brand. They scrimp on frame materials, and they use only low-end components. "Shimano" parts are often toted as a positive thing, but they don't tell you that Shimano makes an entire range of products, from what the pros use to complete and utter crap. Guess which end of the line-up is on a department store bike that retails for less than an upper-end derailleur alone?
One more thing to think about: If your Wal-mart bike has a problem within their warranty period, they will replace it. They do that because even after giving you a free replacement and scrapping the original, they still make money. Yes, as cheap as the bikes are, you're still paying twice what it is really worth.