It is my understanding that under the DMCA the security measures themselves are copyrighted works and breaking them is a violation of the DMCA in and of itself. That’s why breaking DRM, even if it’s to access public domain works is still illegal.
https://www.law.cornell.edu/uscode/text/17/1201
> (A) No person shall circumvent a technological measure that effectively controls access to a work protected under this title