Technically, homebrew shops don't have any alcoholic beverages in them at all. They have barley, hops, yeast, and various other bric a brac. It's the same theory that head shops can sell bongs under. You don't know what the customer will do with it after the items have been purchased.
I've always wondered if someone underage could actually buy homebrewing supplies. I'm sure store policy would say no, but I mean legally if they could or not. After all, there's no law against buying grain is there?