no, a bartender is always obligated to verify the age of people who enter their establishment if in doubt, regardless of how they pay. You won't get off the hook selling booze to minors because they pay digitally.
And that is actually very reasonable for the exact reason I pointed out, if it was MC's responsibility they would have an immediate excuse to snoop into my purchasing history for every minor transaction. (which they already have to a far too large degree).
If an online liquor store was systematically ignoring ABC laws and shipping booze to underage customers across state lines, MC would probably act in a similar way.
MasterCard has nothing to do with verifying the legality of the transaction - pornhub, or whoever, should do their due diligence to validate everything is legal.
Vimeo
Snapchat
CashApp
Venmo
Every single one of these companies has trafficking minors problem. All of them dwarf pornhub's.
In your example, if the underage drinkers pay using MasterCard, then it's MCs job to check, as they're doing to pornhub.
MasterCard should stay out of it, as they do in other situations.