In the motion for reconsideration, the movie companies argued that Cox and other ISPs are not just mere conduit providers under DMCA §512(a); they would also fall under DMCA §512(d), as they can remove or disable ‘references or links’ to infringing content.
If this argument succeeds, a DMCA subpoena would be valid, as these do apply to services that fall under DMCA §512(d).
The movie companies used various arguments to make their case. For example, they argued that IP addresses are in themselves “references or links to infringing material” which can be disabled through null-routing. In addition, Cox can respond to takedown notices by implementing filters or blocking ports