Kind of black and white. I think everyone agrees to not chat up strangers sitting reading books, listening to music or whatever, but if you're idle, what's the harm in saying "No thanks, I don't want to chat" or something similar if someone asks you something?

And yes, of course don't try to speak with people who obviously don't want to be spoken to. Quick way to find out, is to ask "Can I ask you a question?" and then you leave space both for the people who don't want to chat, and the ones that do :)

I used to judge that based on people's faces, but the faces lie a lot, and some people basically default to looking pissed off, while they can be very warm people, and also vice-versa, so in the end asking up front seems to be the nicest way for everyone to be OK with it.

What types of social interaction should be permitted in society?