In my reasoning, it's because nobody's feelings truly matter outside of themselves. Feelings start and are mostly just chemicals in the brain, so why should I be concerned that the choice of random noises I made to a friend cause a chemical change in somebody that overheard? Especially once emotions are active, it's up to the person experiencing those emotions to decide what they do with their feelings. She didn't take long enough to realize she had the power of choice to engage and continue conflict or not. All that was in her mind at that point was, "Nope. Don't like that. It ain't right." And then chased her immediate desire to correct that which she believes is wrong.

Long story short, in this specific case in my opinion, the final ball was in the mother's court to decide whether or not she wanted to stay angry with a person that did something she didn't agree with. She chose to engage someone that obviously lived by a different set of rules and expectations than she expects and to her it was this person that was wrong, not the two of them having different life experiences that led to their actions.

I'm not saying use retard because people don't matter. I'm saying that if you say something in jest with zero intent of harm and that thing happens to unintentionally offend a stranger, it is up to that person whether or not they'll just write you off as not having known better or choose to engage and correct another adult. And if confronted, the power then goes to you to defend your actions as acceptable (which they won't ever be to her) or apologize for unintended offense and walk away.