Bus etiquette and smelly tramps

What's the correct form for changing seats on a bus or train? So frequently I see passengers who're sitting uncomfortably next to another member of the public, squirming and shifting from side to side, then relentingly giving in to their inner urge and moving to another seat. Whether they're just trying to get a better view out a window or whether they dislike the smelly, unwashed looking character next to them is not clear. However, there's never really any need for explanation, they simply move to their newly selected seat, furtively glancing from side to side as they go, as though inviting other passengers to challenge them. It's not normally those who're obvious freaks that are subject to this motion slur, but those who appear only slighly abnormal or weird, the kind of person you don't actively dislike, but merely feel uncomfortable being near.
I avoid the whole problem by not sitting anywhere when there's little room on my chosen method of transport if I haven't sized up the potential nose tormentors. There's one thing to be said about the Great British Public' - that much hailed institution - and that is that the vast majority of them simply smell bad! Hideous is an understatement, a level of sewer revival not experienced anywhere but in another supposedly wonderous tradition, the public library. These are of course rarely used by real members of the public, but mainly by bums and homeless people who need somewhere warm and enclosed to spend their days, providing the ideal opportunity to spread germs as widely as possible.
This may sound like a rant, but it's not meant to be one, but the public get so damn annoying, every so often you just want to be able to only deal with rich, well behaved, polite and clean people. The middle classes are normally at least three of those, so let's all hail the middle classes!