That code is protection for YT TOS, not copyright law. If YT want's to use dcma in this way it should claim copyright of all posted material and see how that works out for them. Even if it the video's were copyrighted, it could be still be downloaded for fair use purpose rather than infringement. RIAA is using takedown's to protect google ad money, which is not the intended purpose of that legal action.
Youtube is not the copyright holder, the RIAA's members are. They have licensed their content to Youtube.
Youtube stores cache files to a user's computer, obfuscated so they can't be easily copied or accessed, most likely as part of the terms of their license. That is legally sufficient to constitute a copyright protection scheme.
Material is covered by fair use and can be used legally in that way. Takedowns are for copyrighted material being distributed to provide immediate relief, the yt-dl projects code is not owned by riaa therefore riaa is abusing the legal mechanism.
