I'm not from USA, but I fail to see why this should be illegal.
It's clearly <designed> for searching for illegal content.
I mean, you have sites called ISO hunt (legal ISOs? yeah right, sure they exist, but clearly that's not what it's designed for) thepiratebay (golly gee we didn't intend for it to be used for pirated content!) and all sorts of other sites. The fact is, if they are, as you said, search engines. but people upload these torrents from their own PCs and it would make sense that, if these sites were actually trying to remain legal, to actually filter those torrents.
The whole "well it's just searching for the torrent files not the actual copyright data" is simply a level of indirection intended to circumvent the law regarding the issue.
A Torrent, the protocol itself, and the contents of the file, aren't illegal. But to compare these torrent search engines to google is a bit naive. These torrent search engines are quite clearly
designed to return hits for illegal torrents, and there is very little that the administrators do to prevent it, which makes them guilty of "conspiracy to commit criminal copyright infringement" (in the U.S, at the very least, anyway).
the reason people believe that having torrents that point to copyright data do not subject Torrent search engines to secondary copyright liability is because of a Case files by Universal Studio's against Sony, in which they claim a "precedent" was set:
the Supreme Court found that Sony's new product, the Betamax (the first mass-market consumer videocassette recorder), did not subject Sony to secondary copyright liability because it was capable of substantial non-infringing uses. Decades later, this case became the jumping-off point for all peer-to-peer copyright infringement litigation.
However, One has to realize that this actually doesn't set any sort of precedent- the Betamax was a physical device, it's use for illegal activity put the maker at legal odds no more then smith & wesson are held responsible for violence and deaths relating to use and misuse of their weapons.
What a Torrent Site would be akin to in this analogy is a weapon manufacturer that provided instructions on the best ways to kill people and commit crimes with their products. Now, not strictly illegal, but the intent is clear- these weapons are designed for commiting crimes. If such a weapon manufacturer existed and their weapons were used in a murder, they would be guilty of conspiracy to commit murder just as much as an accomplice would.
