A german court ruled youtube's obfuscation of video request signatures is an effective technological protection measure. YoutubeIE._decrypt_signature in youtube_dl/extractor/youtube.py bypasses this.
At least with CSS a key needed to be found by means separate from actually watching the DVD and wasn't shipped with the user in the same package as the video itself.
Because your browser also runs the javascript that displays the ads (unless you've also blocked them, which I think could also be considered a "circumvention measure", IANAL).
That would work in a livestream very well, but not in a video. Imagine everyone getting adverts in the voice of live streamer in the different times of the livestream, and live chat getting time-dilated to compensate, without the streamer itself noticing.