Merchants are assigned "merchant codes". Your bank can't tell specifically what is getting purchased, but they can tell that it's a grocery store purchase, gasoline store purchase, entertainment purchase, travel purchase, etc. It's how they're able to give tiered reward categories for different purchases, or allow redemption only for specific categories.
AFAIK, there's no "gun store" (or anything comparable) category.
re: credit cards & liquor - credit cards can't be legally obtained here without being age of majority, so I'd say liquor salespeople should have a stronger argument, since a minor can't actually use a credit card to purchase liquor without performing credit card fraud in the first place.