It asks you when you first connect the router. I suspect most people you have contacted are likely to have subconciously (or thoughtlessly) opted in.
If you let me know the URL, I can test from here as I'm on the end of a Sky connection without the opt in blocking so that would confirm if it is that or not.
A lot of people get technicians in to install the hardware, their default mode is to just next,next finish until the router is ready. Chances are it's the technicians themselves turning on the blocking. Not in a malicious or tinfoil hat way, just out of a desire to get the job done quickly
If you let me know the URL, I can test from here as I'm on the end of a Sky connection without the opt in blocking so that would confirm if it is that or not.