Part of your answer is that the person you're interacting with may never have had any interactions with a person who stutters. Or they may have, but when listening to or interacting with that person and hears and sees a person who is a stammer, he or she might not have heard of the word stutter. They see a stammered as, why are they talking differently from me, the non stammer? May also feel rude to ask that stammer why they talk the way they are talking.