I'd imagine it has something to do with the fact that its closest competitor doesn't carry albums with a Parental Advisory label.
Green Day isn't the best band in the world, but they sure sell a heluva lot of albums around the world. Walmart won't be selling Green Day's next album.
It cracks me up how in this economy, a pretty advanced business is using questionable content as a decision maker for what kinds of goods it will sell.
Furthermore...how can they sell movies like Scarface and video games like Grand Theft Auto right next to the music section?? I haven't shopped at Walmart in 10 years I think. I don't miss it either.