I think if it does come from that, flowersintheatic, it's a shame, and it's not practiced by all designer stores in the slightest.
When I was in college, I was in DC at one point staying at the Watergate for a conference. One of the other girls from school and I went walking along a series of shops nearby which happened to include a Vera Wang store. We weren't dressed super-nicely, and I think it was apparent we weren't looking seriously, but we were still treated extremely politely when I saw a dress I fell in absolute love with in the window and went inside to look at it more closely. (Southern Honey and I weren't engaged at that point, although we did get engaged about a year and a half later.)
The treatment we received that day actually led me to strongly consider buying the dress once I was engaged -- if it hadn't been so darned far out of the budget, I probably would have because I managed to track one down at a local boutique.
Honestly I think it's lack of training, snobbery, and just plain poor salesmanship, or some combination of the three.