My only conclusion can be that this is deliberate, and that they get enough extra money to be worth it. There have been similar stories going back decades - their predecessor company, NTL was notorious for things like this (e.g. at a student house in 2003, we moved out and they “cancelled” but kept charging for months, and didn’t pay it back for nearly a year).

They had a terrible reputation even then, but it was that or Sky.