Martin, I think the issues here are that 1) headphones are different from looking at a smartphone, and 2)women get this kind of behavior all the time, when men tend not to.
I see your point about people not interacting as much as they used to —but to me looking at a screen is very different from listening to headphones. If I’m listening to a podcast or music I like I wouldn’t want to be interrupted by someone hitting on me. (I’m also a man, so I don’t get this problem — if it happened to me on a regular basis, like it does to this author and many women, I’d be very peeved).
By all means feel free to start up a friendly conversation if someone is looking at a screen on a bus …but maybe first ask yourself “if I were this person, would I want this conversation to happen to me?”