how can youtube.com allow users to upload/download tv’s, music videos, and be legally ok? (see details)?

I hope I can get some detailed explanation here, because I am planning to run some podcasting business soon.

We all know youtube.com is legal. but how can they make the site legal?
I am just wondering what they did in their terms and conditions that keep the site (company) NOT responsible for any legal actions.
The site allows users to upload and download video clips originally coming from TV, MTV, movies, which we know are NOT legal.

What if a TV network or a record company exercise their rights, and take legal actions?
Whose responsibility is it? Youtube or the user who upload the clip? Or, the viewer?

I think youtube is BIG now and will become bigger, so they must have done enough to keep themselves away from intelletual property lawsuits.