We can't prevent all children from getting beer, but we can prevent most of them without compromising any adult's privacy. And everyone is ok with that state of affairs and the trade-offs. No one's calling for internet-connected beer cans that make you take a selfie before you can open them.
> we can prevent most of them without compromising any adult's privacy
But we don't. Even with in person age/ID checks the clerk will often enter some of that data into the store's system and then who knows what happens with it.
> the clerk will often enter some of that data into the store's system
I've only seen them enter the date of birth. No identifying information. If they record the ID itself I'd recommend going to a different store. Or ideally, writing your legislators to have the practice banned.
Depending on the size of the town, date of birth could be used to severely narrow down and target a specific person.
If one suspects a partner of buying alcohol and could convince or coerse the clerk, or even just peek in the book, and see the partner's date of birth written there, then that is good enough proof for many people and many purposes.
https://news.ycombinator.com/item?id=46447282