Youtube already has downloadable videos on few accounts. I'm sure this has been debated long before youtube became popular.
If they were to ever release a feature allowing users to download videos, it would probably put a huge dent in the servers, and not to mention the potential of users breaking copyright laws. Here comes my first response: stealing. If that were to occur, who is to blame here? Youtube. For allowing this to happen.
So don't expect this to happen anytime soon. Show me an updated news from youtube and then I will believe you.
As for the other sites that allow people to download through ytURL's, they are acting on their own behalf. They aren't endorsed by youtube and it is up to the person [e.g. you] to break a copyright law, via download.