Re: So what?
There is, actually a contract section which applies. Unfortunately, there are two terms of service documents for YouTube, and I'm not really sure which of the two versions are actually in force. (EDIT - they might have sorted this out, finally)
https://www.youtube.com/t/terms
https://www.youtube.com/static?gl=CA&template=terms
Part of 6.c. of the second link ... " You also hereby grant each user of the Service a non-exclusive license to access your Content through the Service, and to use, reproduce, distribute, display and perform such Content as permitted through the functionality of the Service and under these Terms of Service."
The trick is the, "through the functionality of the service".. so if you download the video in order to then use it in one of your videos which you then upload... you're out of 6.c. because you've gone outside the service in order to mash up the video. So none of this is very straightforward.
I recorded a video on the problem here - https://www.youtube.com/watch?v=7ABqh04n8lo - but it does appear that they might... finally... have actually synchronised the two terms of service.
ie. even though your uploaded work is copyrighted the instant you upload it... the simple act of uploading it to youtube, automatically grants every other youtube viewer the ability to use and reproduce it.