If you are paying by credit card, it is a condition of the terms of their merchant agreement with Visa and Mastercard that additional ID is *not* asked for. All that should be requested is the credit card and if they authorize the transaction then that is enough. I've refused to show my driving licene when asked for it in Walmart and Canadian Tire because of this. Why am I bothered? Well, someone could skim your card and if they have your personal details (address, dob) then they can very easily use your card fraudulently.
The 'being old enough to buy liquor' is a different issue. I'd still rather go elsewhere to buy it esp. if it is for obviously old enough people. Maybe understandable for borderline people who don't appear old enough but even then, scanning it rather than just checking it is too much of an infringement.