During my long-ago employment at McDonalds, nearly all the staff was under 18, and so ineligible to be handling alcoholic beverages, even 3.2 beer. Many American states are quite fussy about this sort of thing--I've had a younger cashier at a grocery store call over an older to scan my carton of beer.

Now I guess there are more of the over-18 or over-21 employed at McDonalds, but it might disrupt the flow of ordering if all orders with beer had to go through particular registers.