Fair Use is about using copyrighted content, which is widely shackled behind DRM. Any court can easily see that in order to exercise your right to use that content in a Fair Use sort of way, you must break the DRM. Therefore, the existence and availability of DRM-breaking tools is a necessary condition for Fair Use to be exercised at all in conjunction with the modern media landscape.
IANAL
Could you cite the law you're referring to here please?
The UK CDPA as amended to follow the EU's Marrakech directive seems to say anything that prevents you from exercising your Fair Dealing rights to make content accessible for disabled people is void if it contradicts these rights. This seems necessarily to allow for circumvention of DRM (for people with disabilities and specific registered companies) but that also appears to mean production of circumvention means needs to be legal otherwise such accessibility will be impossible.
It's absent because RIAA's intent is not stated. They did a blanket takedown, unprompted. As far as I can tell they never requested any particular modification. IIRC the only hint that it might be related is a mention that the rolling cipher algorithm that YouTube-dl "circumvents" was ruled to be DRM under German law.
However, the bulk of the DMCA seems to be leveled at the marketing of ytdl as a circumvention tool, citing unit tests containing metadata referencing RIAA-owned content (unit tests, apparently, are now part of 'marketing,' I guess.)
This is definitely untested in court but I won't be surprised if it is indeed part of marketing. The problem with the tests is that they do download the video, even if it is a small amount and since ytdl does not reject the video for downloading at all it is technically infrigment, probably without a valid fair use defense. If ytdl has actively rejected that (for example if the test units are specifically to prevent downloading those types of videos), they may have a stonger defense against RIAA claims.
It's a right because where it applies there is no tort. Like an allowed right of way over private property. Yes, if a you have someone abusing your rights by filling frivolous suits then "it's a defence", of course it is they're trying to assert a right they don't have.
Such needless couching of public rights in an authoritarian way is really offensive to the purposes of copyright, which is granted by the public - the demos - to private parties. It's not a natural right, and so yes, under Fair Use there is no right being infringed that a valid claim of tort can be made for; so one does have a right to do those things.
https://www.nytimes.com/2006/10/09/technology/09steal.html
Two years later, their case was dismissed :
https://www.generation-nt.com/drm-stopdrm-dadvsi-justice-loi... (fr)
They were considered as 'irresponsible' due to 'either psychological issues, force majeure or legitimate self-defense".
(Note that they even seem to have shared the DMCA infringing software on their website.)