> Sounds like it's not compulsory though? The ISP is perfectly welcome to lay their lines via private lands (having negotiated permission with each landowner, naturally) if they'd prefer.
Streets cut cities into small islands of private lands. Therefore, you cannot reach anyone without crossing the public right of way.
Perhaps mere crossing should be treated as de minimis. But when you're closing and digging up the public street for days at a time, or perhaps even running your lines through publicly-maintained conduits and accessing them through publicly-maintained manholes, you're taking a lot more from the commons than that.
Streets cut cities into small islands of private lands. Therefore, you cannot reach anyone without crossing the public right of way.