So being an illegal content link index (categorized magnet URI lists) is not illegal ? honest question. Isn't that why Google was forced to unlist torrents from results, otherwise they'd be in the same position as TPB.

It can be considered contributory copyright infringement if you're knowingly linking to copyright infringing material.

In the US, abiding by any DMCA takedown notices is enough to be fairly certain you are not going to be in trouble for contributory infringement. That's how Google covers itself.

What about Youtube? It streams huge amounts of illegal content everyday, and it's not just indexing it, but actually hosting the content. Frankly, there would be no YT as it is today if it wasn't hosting tones of music videos illegally.

Agreed. There was a time when they strictly enforced blocking of new and removal of old illegal content. My account was flagged many times for tiny snippets of classic 80s movies[1]. That made YT stay outside legal problems. But since there's been a new wave of 'full movie' uploads so I don't know where they stand.

[1] I'm sure copyright holders benefited more than suffered from this free advertisement of love but anyway.

The problem is that when you start to make things like that illegal, you are actually just banning numbers.