Probably because its dangerous to democracy to silence people who question the integrity of elections. It sends the impression that there is something to hide. And people who suspect that the election was stolen will take this as confirmation of that suspicion. Since they won't be allowed to use that platform to raise their suspicions, no one will be able to respond to them to allay their suspicions. Then as time goes on confirmation bias will lead them to become entrenched in this belief. Eventually this belief (that the elections have been stolen) will lead to the perception of a loss of legitimacy for the government, and consequentially public servants will find it more difficult and dangerous to do their jobs.

How is YouTube protecting democracy? It seems to me its action silences a group of people in a democracy.

Well, I could simply point to the entirety of Taibbi's article. Or even just the headline and sub-headline: "[It's] Un-American, Wrong, and Will Backfire. Silicon Valley couldn't have designed a better way to further radicalize Trump voters." Many of the points I might make, Taibbi did so in his article.

For the sake of novelty, I'll make a different point: I see one way in which YouTube may have promoted democracy: by making their odiousness more clear (and in a public, "everyone knows that everyone knows" way), they may have encouraged quicker production or adoption of alternative platforms. This seems unlikely, because websites like them have done lots of crappy stuff before, without usually causing much effect; but it is possible that this may be seen as enough of a "They've declared war on the entire right wing" to motivate a significant migration. Two partisan platforms is better than one, for democracy and just for competition. (Better yet would be either a platform that has made some kind of enforceable and very-painful-to-break commitment to neutrality, or some kind of decentralized system that no single company or party can decide to censor. We may get there eventually.)

"Corporationism is above socialism and above liberalism. A new synthesis is created. It is a symptomatic fact that the decadence of capitalism coincides with the decadence of socialism. ... Corporative solutions can be applied anywhere"