I think those of you who can't understand that it isn't common sense to a mother you witness swearing & hitting their toddler that that's inappropriate are forgetting that there are so many other factors at work in that mother. She may have come from just as abusive a background, and know no other sort of culture or way of behaving, so will not have it ingrained in her how to treat a child nicely (for those of us to whom it comes "naturally" were probably loved & nurtured well in our own childhoods). Plus, there may be all sorts of horrible things going on in her current life, an abusive relationship, no money, a child with behavioural problems, etc etc.
I can't help judging parents myself when i'm out and about, but I don't automatically assume they don't have any common sense. I assume they have been messed up themselves as kids, and have not been educated about parenting or do not care to educate themselves about parenting etc.
In my own experience, I used to work with people with learning disabilities and I think a lot of my parenting skills- patience being one of the most important imo - have come from having done that job. As part of the job I did courses on personal development etc so I also know a bit about the psychological side of things - attachment theories & all that. I had a chance also to work through my own issues and so was emotionally more healthy and able to deal with dd in a detached way iykwim (ie not loading her with my own issues if that makes sense).
Added to that, re simple manners, saying please & thank you, I must say I followed the example often that a nanny friend of mine set, and tbh i don't think it comes naturally to expect a little toddler to say please, thank you, thanks for having me, etc etc, or to have table manners. It wouldn't have occurred to me if I hadn't witnessed my nanny friend teaching the children she looks after those manners, and I remember one frustrated conversation I had with her possibly even before dd turned 2 "I don't understand how she is ever going to learn to say please off her own bat!" and my nanny friend said, "They do! Just keep at it!" And she was right, and she had that experience of working with lots of children from young ages to know that things do work. I'm sure it's easy for parents to "get lazy" or just not realise all those little things, consistently taught, really are worth doing from an early age.