We just returned from a nice stay at POR, one night on the way from MK, there was an accident and all traffic had to be diverted turning a 14 minute bus ride into 45 minutes, this bus was full, there was a man holding a child, he was offered a seat when he got on from an older gentleman and declined. About hallway through the bus trip he and his wife had to start passing the sleeping child back and forth, no one had expected the trip to last this long. People were miserable, myself included, being 5'2" holding on of those straps for that long is not too comfy!
The lady standing next to me said something to a teenage girl to give up her seat, the girl was about to oblige when her father stopped her and said "it is not our fault that they brought a kid," while a valid point, it still saddened me a bit. These were not normal circumstances. Once the bus was able to get moving, the driver clearly was trying to make up time and took the turns like a nascar driver, people were falling into each other. My arm and shoulder are still sore.
While times have changed, and I do believe you should never expect a seat unless you take the steps to get one, I am able bodied and I will still give up my seat if someone needs it more. I will not judge anyone for not giving up one, when my son was 15 we were in wdw, and he was recovering from 2 broken vertebrae, I always gave my seat to him and got many weird looks from people.