"Preventing" is not the same as failing to enable. As far as I know, there's nothing that youtube has implemented that prevents downloading. It is simply inconvenient as a result of not being the primary goal.

There's an encrypted signature in the response from YouTube that youtube_dl decrypts in order to request the video content.

Perhaps it's not a super-sophisticated technical measure if youtube-dl was able to defeat it -- but it's a technical measure nonetheless.