Think about it this way, if you buy a computer from Best Buy, and you have problems with it, do you go to the sales person you got it from to fix it? No, they don't deal with that, you go to the Geek Squad department since they handle repairs. Or if you had some issue with the store itself, instead of going to the sales person, you go to the Customer Service section. In a T-Mobile store, everyone there is a sales person, there is no customer service or tech support. There is no one to go to because T-Mobile handles all of those things over the phone.
I'm definitely not saying that's how it should be. But that's how it is because T-Mobile doesn't care.