This happens to me at self-checkout at Safeway. I guess I can get somebody to come and check my ID, but seems unnecessary.
If the person manning the self-checkout operations thinks you look awfully young to be buying alcohol then they can always demand to see your ID.
Simple.
Far more harmful is self-checkout without a discount for free labor or treating your customers like criminals when they leave the store with their own property.
You seem to have missed the part where I do not want to ask permission of the government for every little thing. I'm well into middle age and do not appreciate having to seek governmental approval for what I consume. I am fine with some products like alcohol being subjected to taxes to balance out negative externalities. I am not fine with the idea of showing my papers on demand.