I'd love to be wrong here, but there are two separe things, content license and general YouTube terms of use, I think the terms prevent you from downloading any content, the fact that some content is licensed CC has to do more with being explicit, perhaps for displaying publicly or just informative, and I want to think they are leaving a bit of gray area for non abusive use(tons of downloads) , but you can see there is no download button, and if you build something that depends on it to certain scale they could go after you with terms of use, not license.

If you can prove me wrong with some link to where are we covered legally to download CC content from YouTube, it would make me very happy.