Ignore the legality of it for a second and just think what would happen when you have a dinner party that shows up where some people have different phones. The groups would immediately see the price differences.

Also, the QR menus I’ve seen don’t connect to the actual ordering process. Waitstaff come over to get the order, then they punch it into the POS device that tallies the bill. So maybe they could have dynamic pricing day to day, but not from customer to customer