Reading (A) and (B), I cannot help but notice that it doesn't matter whether the primary purpose of the circumvention is to infringe copyright. It is enough that the software/service is primarily about circumventing the "technological measure".
The technological measure itself doesn't even have to be primarily about preventing copyright infringement. It suffices that it effectively controls access to relevant works, even if it's not its primary purpose.
We could even argue that YouTube's restriction are primarily a means of getting people back to YouTube, with copyright enforcement being only a secondary concern. Sadly, this argument would be irrelevant with respect to (A) and (B).
It does, "ordinary course of operation" means "the most obvious use" and they took the tool as circumventing, looking at the test cases that are intended to succeed in circumvention.
No, really, it doesn't. It just makes the RIAA's case that much easier, makes YouTube-DL look bad. (A) and (B) above however are broader than that.
It doesn't matter that the technological measure is primarily about restricting access to protected works. It only matters that it effectively does restrict that access.
It wouldn't even matter if Youtub-DL is only used to access public domain videos, because their primary mode of operation would still be to circumvent the technological measure (assuming that measure restrict access to all works, not just the protected ones).
A "new" tool branded as a way to download public domain videos hosted on various web sites such as YouTube, (let's call it "Public Domain Tubes Archiver"), would still infringe on (A) and (B) if downloading those public domain videos primarily involved going around a technological measure that happens to restrict access to (unrelated) protected works.
Right, it gives creators freedom to build what they want. In this case, YouTube. The sword cuts both ways.