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

I can't let you know the URL, but http://medfirstalert.com/ is one of the other seemingly okay domains on this IP

That's fair.

The domain above resolves fine with and without the block: http://i.imgur.com/AIqtrxF.png

DNS cache was flushed between each hit.

If you have an engineer set up the router, they may just click though the screen as part of the set up, or bypass the traditional set up to save time.

Probably because the government told ISPs that opt-in should be the default.