Assuming that this new Trump administration policy goes through, they could block any website they wanted to. It gives them the power that the Chinese government has over speech, frankly. If you ever spend time in China, you'll notice that the Internet's very different. You can't visit any Tibetan independence sites; you can't visit Taiwanese sites; Wikipedia is blocked; Facebook is blocked. It's a completely different.
After this there's nothing stopping an American carrier from doing the same thing based on their idiosyncratic preferences, or maybe based on what the government tells them to do. Because the Americans have this blind spot toward private censorship, let's say the Trump administration says it wants you blocking all these sites that are saying nice things about North Korea, and everyone says, yes, why should we hear good things about North Korea, let's block all of those sites. They say that they agree that these sites are irresponsible; they're helping the enemy.
I'm not sympathetic, obviously, to the North Korean position, but I am sympathetic to free speech. They have the power. What you're hearing from the ISPs is basically some version of “Trust us”. As the last member suggested, I think Canadian ISPs are still bound by the telecom act, but for the United States, after this rule there are no holds barred, and it's open season for private censorship.