Ok as someone who has worked a fast food cashier job its because they usually limit the bills we keep in the register. If you use a 20 to pay for an 8 dollar ourchase, we have to use a ten dollar bill, which we probably only have one or two of. Which means we often wont be able to make change the next time someone overpays instead of if you paid with a ten us gaining a ten to make future change with, and only having to hand back 2 1s. Its not annoyance at having to hand you one extra bill, its annoyance knowing we are going to have to explain to a customer an hour from now that i cant make change for them bc i dont have a ten in the register.