if serving the infohash was a DMCA violation copyright holders would be thrilled to tear down every torrent site in existence. You can't link to the material, but the infohash isn't that either. It's conceptually a link to the torrent. You're partially right about DRM though. Software to directly circumvent copy protections is illegal under DMCA. But talking about the process is still not illegal, and that has led to some really clever workarounds like the DeCSS Haiku (https://en.wikipedia.org/wiki/DeCSS_haiku)