Your ID is going into some kind of database. I would also be upset that someone would record my ID details without my permission. A couple times at Walmart, the cashier demanded that they put my birth date into the computer for buying booze. And my response was to put my items back in the cart and go to another cashier. One old fart said he was required to do that, always. (Liar.)
I don't mind them seeing my ID, even though I must look well over 40. I do object to them tracking me.
If their liquor buyers were evenly distributed between 21 and 66 (just assume for a moment), that 45 year span would give more than 16,000 unique birthdays. The city has a population just over 10,000. Divide by some number to get the number who shop at Walmart. Statistically, the odds are very high that they can identify me just by my birth date.