Indeed, and he could join in the conversation and have a very nice conversation and make friends. But he is not doing that because he is working. It is not likely that he would join the conversation if he wasn't working, as such the joke does not make much sense, but it is definitely something he could do, which could indeed be a nice conversation for the three of them, or it could be terrible for some of them.

I don't see why you put it down so black and white. Is it prohibited to start a conversation with strangers on public transport? How do you meet people? I often talk to strangers, it's generally pretty nice.

Granted, the last few years I didn't take public transport very often: I mostly walk, but when I do I generally start a chat with someone.

Yep

Gosh, just laugh or give it a ↓ if you don't like it