They tried that in Denmark a decade ago (or more). Enabled a "public wifi" on all home routers to provide wifi coverage for that company's customers everywhere.

They gave every assurance that it would be secure, completely separate from the users own network, and the bandwidth consumed would be added on top of whatever the user had a contract for.

It took a couple of days after launch before somebody managed to gain access to someone's private network from the public network and they shut it down again and never opened it again.

Anyway, I may be spoiled from living in Denmark, but I'm using my own router, which is an option when ordering, so I'm fairly certain nobody is sharing additional wifi hotspots.

And it's not just me. The majority of people I know use some kind of 3rd party router with their ISP. Granted, some of them just use whatever the ISP sends them, but even then I haven't ever seen any additional wifi networks, and the ISP prints the router admin password on the box itself, so you can poke around if you like (though the ISP has a backdoor, which is also why 3rd party routers are a thing).