Copyright law does apply to big numbers - any computer file is effectively just a really big number, and can be copyrighted, and this specific kind of cryptographic key, even if not copyrightable, still (potentially) counts as an illegal DRM circumvention device under the DMCA (and equivalent laws in other countries). Illegal numbers are a legal grey area as there's no case law demonstrating that they're legal yet, but it's expected that if it ever ended up in court, they'd be determined to have been legal all along. Wikipedia lists a whole bunch of them on their illegal number article as they're sufficiently confident they're not going to lose a legal dispute. Big publishers benefit more from there being a legal grey area they can threaten people for operating in than they would from the potential gamble of getting lucky and having a court agree with them while risking the court might disagree and set a precedent they don't like.