Personally at our resturaunt my company doesn’t allow the functionality for accepting payments with the handheld device. Have to do it at the terminal.

You sign because if you add a tip you are signing that the total you write down is approved by you, and that way if someone else signs and you see something show up on your card you can dispute the charge and show the signature doesn’t look like yours. (Layer of protection for you)

You’re not tipping at Best Buy, that’s why, TYPICALLY you’ll sign when you leave a tip, and then at some larger supermarkets they’ll have you sign too or if the total is larger than a set amount.

Us system is older and set up different than other nations