Why do I have to pay a restocking fee?

I purchased an iPhone 3G yesterday at an AT&T Corporate store. As it turns out, there is NO 3G coverage in my home town. No one from AT&T even bothered to check to see if coverage existed there. So I'm returning the phone today because it's useless. I was told that I have to pay a restocking fee! Why do I have to pay this restocking fee for a device AT&T never should have sold me in the first place? They're supposed to know the technology, its limitations, etc. Instead they sold me a product I can't use. I'm filing a formal complaint here.