> They've not for Netflix, nor (to my knowledge) does youtube-dl support even the videos on youtube that you have to pay to watch.
And that is the difference.
For paid content the user has entered into a contract and that probably stipulates what clients can be used to access the content. Same as with Netflix. Google could argue the same for non pay-walled youtube content but that would be very difficult to legally pull off I expect - they would essentially be effectively changing the licensing conditions of all that content. If they can't (easily) or won't (for PR reasons if nothing else) enforce it legally, then there is little point trying to enforce it technically.
