politeness is a form of self respect. its also something that has been part of many warrior cultures dating back to pre-BC china. in feudal japan, samurai considered it a high art form and part of their fabric of being. miyamoto musashi wrote about it and the hagakure even discussed it. pretty sure I'd take the words of a guy who fought death duels vs people who get mad when their chicken nuggets are 20 seconds late. maybe the problem is you just weren't raised properly. its not like low breeding doesn't come into play.
and a cashier doesn't have to help you if you're a douche, they can ask a manager to throw you out.