Browsers are not the only user agent out there, and that's not an active measure to prevent downloading, that's a decision not to implement a particular functionality.

There are also reasons that aren't infringing to grab your own uploaded YouTube content. Maybe you switched to another computer and left your portable drive at home. You have the right to access copies for personal use.

That they had the nerve to go for the tool is yet another extension of the war on first sale, and buyer freedom. Also, the people who made youtube-dl are not committing copyright infringement.