Sure, and if I were ever to interact with that human directly, I'd use my politeness. I feel like manners should be something special just for us humans.

That's a very shallow way of looking at things. It's not about manners, it's about perceived bossy tone being subconsciously associated with the machine's owner.

You get that "scan your credit card, human" and you will leave the store with a fuzzy feeling that something was off, someone was rude to you. While some people will try and analyze it, most - won't. They will simply pin this on the shop as a company. Does the company need that? Nope, it doesn't. Hence the "please."

You are quite free to be impolite to the machine.

I'd prefer that the machines continue to be polite toward me, if for no other reason, because that makes it easier to ignore the interface, and push it into the background.