As far as I understand the issue (and I am not a lawyer or involved in this kind of thing on either side), it would be the uploaders that would need sync licenses, not YouTube, as YouTube is a hoster and the uploaders are ultimately responsible. If I understand correctly, YouTube's responsibility is based in the DMCA and is limited to taking things down and having some way of kicking off repeat infringers.
"My, it's a nice collection of songs you have here...it'd be a shame if you didn't accept these pennies for the effort we are going through to give them to you..."
Considering a sync license is a very solidly established issue (just ask Monster vs. Beastie Boys), I think YouTube is really setting the stage for a hard, hard reality check.