First, to the extent that this is true, the companies that are represented at DS don't care that you can find everything at home in your mall. You think they don't know that? Frankly, they don't care if you buy at DS or not. What they do care about is state-dependent impressions. You are at WDW. You are having a great time. You are carefree and loving life. You see a cute pair of shoes in the window of a store. You don't buy the shoes. You go home and see the same shoes in your mall and your memory carries you back to your trip to WDW and how happy you were and how much you loved those shoes. So you buy them. Had it not been for the fact that you first saw them at WDW, maybe you wouldn't have. But your joyful memories influenced your decision to buy. When you associate a positive feeling with the shoes, you are experiencing a state-dependent impression. For the same reason that Earl of Sandwich tastes better at WDW and a Mickey Bar tastes like something more than it is, which is an ice cream novelty that you can buy at any supermarket. The shops at DS are largely designed (from the tourists' perspective) as giant window shopping opportunities to get you to visit their stores and buy things once you get home. Corporate HQ doesn't care if you buy in Lake Buena Vista or Lake Charles. They just want to grab your attention. And more specifically, they want to grab your attention at a time when you are at your euphoric high. It's a huge game of psychology and it works.
Secondly, I am more in this camp. It is a gross overstatement to think that all of the stores at DS are within the easy reach of WDW's complete demographic. Sort of like saying: No one should eat at Via Napoli because you can get great pizza at home. Well, I've travelled throughout most of this country, and I can tell you that most of this country does not have good pizza within a 50 mile drive. There are plenty of places where Papa John's, Domino's and Pizza Hut are the standard bearers.