Here's a statement I got from a YouTube spokesperson:
"Creators have been asking us to launch a subscription service -- so that, combined with user demand, is why we built the service and why the overwhelming majority of our partners, representing over 98% of the content watched on YouTube, have signed up. Videos of partners who don’t update their terms will be made private in the US at launch because we think It isn't fair to ask a fan to pay $9.99 for a service that has less content than a free service."