After reading your posts in the other thread, you seem to be a disciple of situational ethics. You made the point that "no laws were broken" so everything was OK. Didn't the person returning the item represent that he bought it at CompUSA? Oh, just because he didn't come right out and say so, then that's the store's mistake, I guess.
If stores began an third-degree interrogation of each person trying to return an item ("Did you buy this item from us? How much did you pay? When did you buy it?...) you would probably be the first in line to puff up your chest and proclaim outrage over the way "you were treated like a common criminal".
It's too bad that you don't see what is wrong here. You undermined your argument totally when you tried to defend the person by saying if anyone ever stacked coupons, etc., then they should keep quiet. I doubt very much that if you had your car stolen, you'd agree with the thief's excuse that he once saw you shoplift a pack of gum when you were 10, so you have nothing to complain about.