That's... quite a response in defense of a tool intended for breaching TOS and performing copyright infringement. Can you clarify exactly who it is and isn't OK to steal from, again? I'm struggling here.
Does using a different User Agent instead of a typical browser amount to copyright infringement in any jurisdiction?
You can see the terms of service here:
https://www.youtube.com/static?gl=GB&template=terms
In particular, the first three points in the "permissions and restrictions" section explicitly prohibit tools like youtube-dl. I've pasted these below:
The following restrictions apply to your use of the Service. You are not allowed to:
1. access, reproduce, download, distribute, transmit, broadcast, display, sell, license, alter, modify or otherwise use any part of the Service or any Content except: (a) as specifically permitted by the Service; (b) with prior written permission from YouTube and, if applicable, the respective rights holders; or (c) as permitted by applicable law;
2. circumvent, disable, fraudulently engage, or otherwise interfere with the Service (or attempt to do any of these things), including security-related features or features that: (a) prevent or restrict the copying or other use of Content; or (b) limit the use of the Service or Content;
3. access the Service using any automated means (such as robots, botnets or scrapers) except: (a) in the case of public search engines, in accordance with YouTube’s robots.txt file; (b) with YouTube’s prior written permission; or (c) as permitted by applicable law;
YouTube's ToS does nothing to change applicable law, which was my stated question. What YouTube wants is not necessarily what the law says.
Besides, YouTube does not hold copyright on most videos, so their ToS does magically not allow them to claim copyright infringement.