Apple isn't quite in the same position of power as Wal-Mart. Without inventory (which in this case is unique) Apple wouldn't have a store. This gives more control to the labels who can 'hold out' until they get the deal they want. If Apple says "screw off" to Sony/BMG or any of the big boys that's a huge gaping hole in their inventory that can't be replaced.

Wal-Mart on the other hand can stock underwear or TV's or food from pretty much anyone, so they hold all the cards and can pretty much write their own deal.

Still, Apple has some control as they basically own digital music market so I would imagine at this point, the labels don't have a viable 'plan b'.