A healthy dose of legal realism here: doing something that is technically within the scope of the law as written does not fully remove the risk and expense of getting sued over it. The process is that you can still get sued by Viacom for a billion dollars, spend a ton of money on lawyers and discovery over many years, and wind up making the entirely reasonable call of implementing ContentID or the like and settling. You do more than you're legally obligated to do on behalf of copyright owners, but whatever, the people uploading videos bear the brunt of this cost.