You are talking about the "content" bits. These illegal primes do not represent copyrighted/authored works, they are the secret decoding keys (remember DeCSS?) that are crucial to securing certain forms of DRM. As such, they are more like trade secrets. But trade secrets don't have much legal protection, so the industry got wise and helped write some new laws (to call them copyright, among other things) and can now further protect/entrench themselves.
OK, I may have missed that, but not all illegal primes are content, so not all of them are subject to the copyright legal regime. Hence, the DMCA exists to close those gaps by declaring new kinds/classes of numbers to be illegal.
