RT didn't block Youtube. The Russian government did. And up until the minute it did, RT continued to make money from advertising on it.

The reason one would be okay and the other wouldn't be is, to the extent that RT is a "private" enterprise, it's their right to determine what content is on their own platform, just as it's Youtube's right to determine what content is on theirs. It is not the government's right (in a free society) to determine what sites citizens can or cannot access.

RT's actual website with videos is still available in America.