Is 'duty free' a scam ?

I believe the theory is that you don't pay the duty because you're not consuming the "goods" in the country where you're buying them. But as someone else said, you may or not be getting a deal there, so unless you show up knowing what you want to buy and how much it costs "outside", you may be setting yourself up to be "scammed" just because you don't know how much the item is really worth.