I think both "at some point someone else will inconvenience you" and "we should do our best not to inconvenience others" are both pretty obviously true, so the debate seems to be about what a reasonable amount of time taken is, which is pretty hard to say.
I don't entirely understand statements like "it's not other people's responsibility or problem if you have to get back to work". Well, yes, this is true, and nobody's saying it should be a law to only take X amount of time, or that the mother was *more* rude because of the person behind's circumstances, which she obviously knew nothing about. It still seems to me that it's in general inconsiderate to do something unnecessary that makes other people wait. It might not be 'their problem', but it's just nicer in general. I mean, I could just as easily say "it's not my problem that you want to teach your child about money".
Sometimes unforseen circumstances come up, sure, and there could always be some extenuating condition, but without knowledge of anything like this on either side, I still think it's best to take as little time as is reasonable--again, of course the definition of reasonable is going to be. I mean, to me, all of the "well, it could've been someone who was going slowly due to an impairment etc" isn't really relevant to *this* situation, which wasn't someone with an impairment.