Port Huron, Michigan is on the Canadian border and a lot of people cross over from Sarnia, Ontario to shop.
The local Wal Mart refuses to accept Canadian coins. (Coins that are just rolled up and deposited with the Irest anyway.) If Canadian nickels, dimes, or quarters are found in a cashier's drawer, the total value is deducted from the cashier's pay. In other words, the store managers look at every coin while they're cashing out a drawer.
Since Canadian and US coins now look almost exactly alike, they're even harder to spot. The company apparently thinks that time spent on the clock by managers examining every coin is worth it!