It's funny, I have lived in France for 5 years and never once been ID-ed either in a bar or in a supermarket for buying alcohol. Nor have I ever seen anyone ID-ed even though there are signs everywhere saying you have to be 18 to buy alcohol.
At the supermarket on the corner, I once saw a clearly underage boy, who looked about 11, doing a substantial shop among which was a six pack of beer. The cashier asked him who it was for, he said his dad, the cashier let him buy it. Not sure how I feel about this...but France certainly has less of a social problem with alcohol than the UK.
Every time I go back home to the UK, I get ID-ed somewhere or other, usually in a supermarket (I am 31). And since I'm so unused to it, I never have ID on me.
This new development of parents who are clearly way over the age of 18 being refused permission to buy alcohol when they are shopping with teenagers or children seems utterly ridiculous.
I don't blame the cashiers though, if they stand to be personally fined or prosecuted. If the innocent-seeming customer actually turns out to be a trading standards checker, why would they take the slightest risk?
It's the body behind it all that needs to get a grip - is it all coming from Trading Standards? Or is it Tesco themselves or what?