What if the website itself doesn't do web-torrenting—in fact, it doesn't even make the magnet links
links, instead just showing raw magnet
hashes as text—but there also happens to be a well-known "unaffiliated" browser extension that does "gradual enhancement" of any site using the magnet-hash text "microformat",
into a web-torrent download button?
The site doesn't link to copyrighted content. It doesn't link to anything, in fact.
And the extension doesn't contain any references to copyrighted content. It contains no content, and is completely unaware of any sources of content. (No explicit whitelist patterns for sites it should run on, etc.)
And—at least as far as anyone can tell—the two things have different authors.
But put them together, and oops! You've got a piracy program! User's fault for putting them together, of course.