A: Although you may speak English fluently, you may not speak it well enough so don’t be surprised to learn it’s the North American who may be justified in calling you impolite!
A request, however kindly stated without “please” is perceived as unfiltered bossiness. What’s lost in translation is that you’re accustomed to asking for something in your language without having to actually add please because politeness is implicitly encoded in your verb endings. English is simple and efficient, but it’s limited because users originate from somewhere else and lack shared values. So that’s a really good reason to remember to say please (and thank you, but that’s another story) next time you’re at the checkout counter.
Here’s the wordy backstory: “Please” is short for if you please, or rather if it pleases you to do this: French si il vous plait, Spanish por favor Italian: per piacere and literally means “you are under no obligation to do this.” So, if you say, pass the peas (not that you have to) the invisible meaning is we (who speak this non-English language) have a mutually agreed upon social contract of shared meaning which would be impossible not to comply with. So when you ask someone to pass the ketchup, you are actually giving an order, softened by adding please.
It’s just one way the invisible hand of culture drives our behavior in ways you never imagined.