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There is a potential cultural reason then, thanks beso and others who mentioned it.
Aww I'm not some free seat tsar :) I don't mean people who have bags on seats or stay say in the aisle seat when there is loads of room, more the people who still do it when the bus is packed full (common round here, especially on some routes and round rush hour) and people have to stand or the bus driver won't let people on as there are so many standing.
I have an issue getting back home after a hospital or GP appointment if it's later on as there is no way I will get a seat from the stops for my bus by there, even though it's only a 10 minute bus ride from home I have to get a bus back into town (20-30 mins) then back out home back past where I came from to get a seat, another 30-40mins so adding over an hour to my journey to get home sometimes.
It annoyed me before the stick but is more noticeable now I need to sit down.
Problem round here is that a lot of the people who do it tend to be more unapproachable - probably why they are doing what they do in the first place when it's really busy. I'm not saying all of them, of course not but there are people who are just disrespectful and selfish.
I always used to go upstairs when I could, leave the seats downstairs for those who needed them and for more space etc...