I used to teach Alcohol Seller's Training for HEB and the whole thing about the kid not touching alcohol is a CYA thing. You see, the seller is responsible for damn near everything that can happen after the sale.

If you sell to an intoxicated person or a minor, you can be held liable for anything they do as a result of the sale. Their receipt ties the sale back to you. Because of this and the fact that people will sue the seller instead of taking responsibility for their actions means that as sellers/servers, we had every right to be overly cautious. If that threat was hanging over you, then you might be overly cautious as well.