They provide all the tools to discover high quality videos and channels. It's called "like and subscribe". If you use those features, it doesn't take long before YouTube shows you only high quality videos. And there's also the dislike button and "Do not recommend this channel again", if you need.

> Freedom of speech in the US...

Freedom of speech is a subject which is much larger than the US constitution. I'm not saying YouTube isn't legally allowed to block sponsored segments. I'm saying that they might not want to because they don't want to limit their creators' speech in that matter. Especially considering how easy it would be to side-step. What would be their reason? They've already made it easy to skip sponsored segments.