Except that opening the manholes is a crime, using the ISP's channels would be at least a civil cause of action, laying such infra requires a municipal permit... The ISP was not worried about such "competition".
Now try doing this in the US, the land of endless red tape, NIMBYs and HOAs. And without having an already dug cable channel. Sure, that's going to scare the probably multi-billion-$ incumbent ISP...
Even YC startups are encouraged to break the laws. The key is knowing which laws you can break, how much you can break them, and what's likely to happen if you're caught. When an illegal good thing is caught, the response is usually to slap on the wrist and legalise it.
A lot of current ISPs did start out illegally too.
Don't splice into other people's fibers though. That's a much worse crime of property damage.
You won't be in front of a jury for "setting up fast internet". You'll get caught climbing into a manhole with electrician tools and charged with terrorism. The jury will be fed a story of how you had expensive specialized equipment on you, so this was a well-funded professional attempt at sabotaging critical infrastructure. You'll have a shitty public defender who will only realise that fiber in this case is about internet not clothing because he first read the file in the taxi to the courthouse. You'll take a plea deal because you can't afford a trial.
The system doesn't work how you think it does, at least not for the people on the ground.
To put it another way, the building of infrastructure should be a monopoly, but the use should be free (as in speech, not beer)
It’s funny that you’d mention this. My apartment fiber has four lines going into my unit, all under AT&T service, apparently for redundancy. I only use one.