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."