It's not. The First Amendment guarantees you a voice, but not an audience. Youtube has every right to tell you to sod off if they don't like your content for any reason.
Of course it isn't. YouTube isn't the government, so it doesn't even apply. My comment was poking fun at people who forget about that.
FWIW, I upvoted you back into the black, since I thought your intention was clear. Lots of people seem to make this mistake, that somehow the first amendment means that private companies aren't allowed to censor you.
Technically, they are not censoring you, they just aren't providing you a free podium.