A lot of times I see this kind of thing happen and I just sit down. I figure "hey, if this guy/girl wants to take up two seats then I guess s/he doesn't mind me sitting on their lap" and I just sit right down. It's no problem, I've never gotten any trouble for doing it. I don't think people are as rude as they are being made out to be here.
This reminds me of when I lived next to Trade Fair on Steinway Street. The space between the aisles are so tight there that two people can only barely fit through. You can't help but bump into people as you weave down the row. Every once and a while somebody get's all fired up with all the "bumping" and waiting in the traffic jam and you see them standing there brooding in anger that people are so "rude" to bump into them etc. One lady in particular I remember went down every aisle of the store yelling "EXCUSE ME, EXCUSE ME" and then I heard her say "NEW YORKERS ARE SO RUDE, DOESN'T ANYONE SAY EXCUSE ME ANY MORE?!". "No," I said back, "in New York you are automatically excused". In other words, if we all went around saying "excuse me" in the grocery store and on the subway for every little bump or mistake in etiquette we'd be saying it incessantly all day long. I only say "excuse me" if I accidently run into someone or something, but not for little bumps or traffic jams along the way.
My point here is that I think the rule is the same on the subway; when some people sit down (and don't tell me that this is a male-dominated problem) they may just by habbit put their bags on the seat beside them, or spread their legs, or lean over into the next seat, or sit with their legs up on the next chair - but that doesn't automatically mean they are being rude. If you want to sit down, just sit down - chances are the people will scoot over (they do for me, and I'm not exactly small).
My motto is that in New York you're automatically excused (unless you really ram into someone or something). So I just make room for people without them saying "excuse me", but also expect others to make room for me without my having to say anything either. I just sit down and it's never been a problem. Honestly though sometimes I forget and I sit there with my legs crossed or something (which blocks the next seat) and just forget. But when someone sits down it's no problem, I just make room.
I'm not making an excuse for people who spit on the seat or whatever, but some of these stories are just the result of some of you not realizing that it's okay to just sit down I think.
I do find it mildly distrubing that it's expected that the males in the car will give up their seat if an elderly person, or if a woman comes into the car. That's both sexist against men and patronizing to women and elderly. I'm against discrimination. If someone wants to give up their seat that's an individual thing and I personally do it on a case by case basis, but any "rules" that say that people "should" act that way are just plain unfair to all involved. I had an old guy yell at me one time for singling him out and offering him my seat. He was right, what exactly am I saying by doing that? He told me if he needs my seat he'd ask for it, but didn't appreciate my making assumptions about him.