Thank you for everyone for your feedback. Some very interesting povs and I appreciate hearing from the parents.
I wanted to clarify that I didn't really feel the mom was rude. That is why I used the word "misstep" in my thread title - I was curious if this was a common parenting thing or a faux pas and if so on what level.
I think the world is a nicer place when we are considerate of others. I was very sympathetic to the mom. Didn't make my ears hurt any less but... *shrug*
Actually, I was worried about the girl because it was such a big meltdown. I really felt her frustration (side effect of being very empathetic). I was bothered by the idea that the situation had to play out as it did, and was pondering how else it could have gone. Maybe it isn't a 100% clear cut etiquette question. There are often "mitigating" circumstances or personal differences that change the etiquette 'verdict.' From what I've read on this board, one person's rudeness can be another person's politeness depending on the context, culture etc.
I find it interesting and timely that many PPs mention that if it had been the, they would have appreciated it if the mom had apologized just to acknowledge that her daughter's meltdown wasn't happening in a bubble. I am currently reading "The Five Languages of Apology" by Gary Chapman. If I had not had my earbuds in and the mom had done one of those helpless shrugs at me and said something like "Sorry, she's having a rough morning - she'll tire herself out. Kids - what can you do?" I would have shared a sympathetic smile and left it at that. Even if she had just made eye contact, rolled her eyes and winced or something, I would have felt a little better about the situation. But the mother was acting just as seemingly oblivious to me as her daughter, which made her appear a little callous. I don't know if saying something to me would have broken the "ignore the tantrum" rule since she would have been talking to me and not her daughter, but I expected her to at a minimum acknowledge that what he daughter was doing was potentially affecting a total stranger sitting inches away.
I agree with the idea that we should try and cause as little disturbance to others as possible as a general rule of thumb when going out into the world. But of course common decency asks us to show some patience and compassion to those who cannot manage to do that all the time.
I guess I wasn't asking about parenting technique or style or rights...merely wondering where one's responsibility to one's child ends and one's responsibility to others begins.
Just so we can all have a better day - which I hope that mother and daughter were able to have.