I see your point but it's STILL flipping. I don't see how you can say your A is wrong and this is right.
I'll show you:
A) takes a $13 PM purchase and deceptively changes it back into $50 credit at the same store (but usually a different location). So you're screwing them out of $37 cash.
B)exchanges a $50 game for a different $50 game, regardless of how much you paid for the game ($13), you're still leaving with a $50 game.
In B, the only moral crime is getting it for $13 in the first place, but if PS2 and Xbox BF2 are both worth the same (which they are), then trading one version for the other is no extra loss to the store.
In A, they're unwittingly paying you for absolutely no good reason.
But on the side of logic, I'd have to say it's really stupid in this day and age for a store to take something back at full price without a receipt. There should rightly be a 20% restocking fee deducted from the lowest sold price w/in 1 year if you don't have any proof of purchase, or possibly not take it back at all.