In any case, a download button is basically like a DVR. It is also an obvious feature that
- is easy to add
- is logically part of any video hosting site
- is wanted by most users
DVRs may be legal according to the law, but youtube and other sites spend an extraordinary amount of effort to make it hard for users to record the videos they are watching. It's basically a technologically enforced ban on video recording for the 99% of users who can't easily use youtube-dl or other tools.