IANAL, but, the issue doesn't seem to be quite that simple. ISPs argue strenuously that they're not actually common carriers (the legal term of art for basically being "dumb pipes") because there is a feeling that it would bring them under FCC Title 2 regulation instead of Title 1 regulation. Whether or not that's correct is still an open question.
But the other way that being a "dumb pipe" comes into play is with the DMCA safe harbor exception (which is 17 U.S.C. Section 512). There, an ISP isn't liable for infringing material passed through its network if (among other things) "the transmission, routing, provision of connections, or storage is carried out through an automatic technical process without selection of the material by the service provider".
So the bit-torrent throttling Comcast was trying a couple years ago might have put it outside of the DMCA safe harbor, but simple monitoring like this probably wouldn't.
The only part of the safe harbor where knowledge comes into play is when an ISP stores files on its servers at the request of users (so, a webhost, or dropbox). In that case the ISP is only protected from liability if they have no actual knowledge of the presence of infringing material.