If that isn’t it I don’t know what the joke is. Maybe Americans are so stupid that they accidentally make the right choice?

I think the m is supposed to stand for miles. I guess the sign is in a desert between usa and canada

It is to stop items from accidentally getting charged to the wrong person. It benefits both people. I never expect a thank you for doing it. If something gets charged to the wrong person, everyone gets screwed. You, the cashier, and this idiot. Now everyone has to wait for the cashier to void the transaction and call a manager over. Congratulations, you played yourself.