> You’re not interrupting my focus for me, you’re doing it for you.
While it may be selfish and pointless, it's the default expectation that in public space people can be spoken to, but it costs something to remove that possibility without also physically isolating oneself in some way. Not all public space is necessarily social, you can be alone in a wooded glen which creates a proximity barrier, but trying to preserve your whole private sphere while being in an otherwise potentially social space removes something from that space.
When I deliberately don't want to chat with anyone, I just take a side street or something. Not always possible, but it's rarely worth it; usually work is the semi-public space I'd prefer unbroken focus.
I do think it's overblown to make some grand statement about this behavior if it's only an occasional thing, but if the default expectation shifts to people hesitating to talk to people only because they might have headphones in, I think we've lost something.
> it's the default expectation that in public space people can be spoken to
It is not the social norm that anyone can be spoken to in public at any time, you are oversimplifying things. E.g. it is largely considered socially inappropriate to strike up a conversation with a stranger on public transit when you’re squeezed in like sardines; we don’t talk to each other to give the illusion of privacy and space. It’s also not considered socially acceptable to have a conversation with a stranger standing at a urinal. There are significant social rules about which adults and children can speak to each other in social spaces. Etc etc
There are and always have been situations where it is more or less socially acceptable to speak to a stranger in public. Headphones is not a new one, I knew in the 90s that headphones meant “don’t talk to me”.