Fair use is an affirmative defense against copyright infringement. Broadly speaking, this means that it is legally irrelevant until you are accused in court of copyright infringement, at which point it becomes relevant.
What happens in Youtube is probably covered by the terms of service (like most people, I haven't actually read it in detail, so I don't know the specifics of how it will play out, but the idea should be generally correct). Terms of service is basically a contract, and contract law totally lets you prohibit one party from doing things that are otherwise legal. So fair use doesn't matter, unless it's written into the contract, which it almost certainly isn't.
The other dimension to bring up is DMCA. This absolves Google of any copyright infringement so long as they expeditiously respond to takedown notices and forwards them to the respective user. The user can object, at which point the original copyright owner must file a lawsuit. Only in the lawsuit will fair use be considered (this is one of the criticisms of DMCA). However, the ContentID system at play in Youtube is not a DMCA takedown system; it is completely independent, and thus DMCA does not apply here in the slightest.
> Fair use is an affirmative defense against copyright infringement.
This is a misnomer.
Fair use is an authorized use, and consequently is “distinct from affirmative defenses where a use infringes a copyright, but there is no liability due to a valid excuse, e.g., misuse of a copyright.” Id. Lenz, 815 F.3d at 1152
What happens in Youtube is probably covered by the terms of service (like most people, I haven't actually read it in detail, so I don't know the specifics of how it will play out, but the idea should be generally correct). Terms of service is basically a contract, and contract law totally lets you prohibit one party from doing things that are otherwise legal. So fair use doesn't matter, unless it's written into the contract, which it almost certainly isn't.
The other dimension to bring up is DMCA. This absolves Google of any copyright infringement so long as they expeditiously respond to takedown notices and forwards them to the respective user. The user can object, at which point the original copyright owner must file a lawsuit. Only in the lawsuit will fair use be considered (this is one of the criticisms of DMCA). However, the ContentID system at play in Youtube is not a DMCA takedown system; it is completely independent, and thus DMCA does not apply here in the slightest.