While I don't particularly trust Google all that much anymore, the fact that ISPs even have an opinion on this is a smoking gun that they're doing sketchy things with DNS data. There is no actual technical reason why they should care if you use their DNS servers or something else, even a private, encrypted DNS service.
Here in the UK sky recently pushed out an update to their firmware which made it impossible to use another DNS server other than theirs. There was a decent amount of push back, I managed to get them to downgrade my firmware but who knows how long that will last before they "accidently" update it again.
Their tech support (the one that's "if we solve the problem we'll charge you") said they could not solve this for me.