I'd imagine that they'd already considered butterbeer to be sold over the counter, I just think that there's probably some sort of process they go through that would make selling it over the counter fairly unauthentic, and the finished product might not be all that great, either. I'm not exactly the expert on butterbeer here, but I, personally, don't see why they wouldn't sell it in bottles to bring home with you, or even give you the option to mail larger amounts of it elsewhere. That's a goldmine just waiting to be tapped.