This is becoming a touchier and touchier subject. Some people are ignorant, others just don't care. Some have medical issues, some people are sitting with their families and significant others. I know if I walk 10+ miles in a park in a day, I'm gonna want to sit down. I will give up my seat, but there are other times where I'm going to keep myself planted. I have some medical issues. My DGF has many more issues than I do, so she would prefer that I sit with her. If she wants me to sit next to her, she's who is important to me, so she wins.
Again, times are changing. As a poster stated above, this isn't mom & pop 1950's. If I'm seated and we get moving and I see a kid or parent struggling, yes, I'll more than likely be willing to do something, but I also work on a situational basis.
You wanna dive deeper into manners, let's bring on the parade/fireworks ethics discussion. How about line cutting? Saving tables at quick service restaurants? How about controlling kids and not letting them run off and having parents screaming across the park to them? Maybe we could discuss how to clean up after ourselves (just because it's a vacation spot, doesn't mean you trash the place because it's "someone kids job" to clean it up). I mean, I was brought up well and also through marching band and such learned that you leave a place better than you found it.
I think there are far worse things that could use a manners discussion than seating on a bus.