They can, but I'm not so sure that "may or may not possess illegal content" is enough for a search warrant - it's true for most of the population after all (running an exit node or collecting funds on behalf of a third party on the other hand is true only for a tiny fraction of the population). Granted, the chances are somewhat higher for IT people and higher still for people that write crawlers, but "we think he might, it's not impossible that he doesn't" is a bit thin, and unless they're trying to go after you for unrelated reasons, DAs don't love to have their asses handed to them by judges.
In many jurisdictions around the world it is enough. In Germany you need a "begründeter Anfangsverdacht" (reasonable initial suspicion) and what's reasonable is essentially up to the judge signing the warrant.
Hell, they used to raid people accused by third parties of copyright infringement (for private personal use), about a decade back or one and a half, but thankfully that stopped now. They would come early in the morning, present you with a warrant that said "based on evidence provided by <third party>..." (i.e. somebody somehow collected an IP address you might have used off of some file sharing swarm), take all your shit and scare your neighbors and quite often your parents because they raided a lot of minors too.
I know two people who this happened to personally.
One guy wanted his stuff back, to which the DA replied that if they got to keep his stuff they would drop the case (I kid you not), and the other guy had his stuff returned about 2 years later, except for his HDDs. And his stuff not only included computers, CDs, DVDs, a printer, but they had actually seized books... paper ones... wat. Neither was convicted of any crimes in the end (IIRC they both had the thing dropped because "minor offense" not worth pursuing).
Turns out that a little googling of the German internet around that time turned up a lot of similar cases and some people claiming the police and DAs did that to get new computers "cheap"...
