For a wiretap, they'd definitely need a warrant. And I think there was a recent court decision that says that police need a warrant to put a GPS tracker on someone's car. What they did here was to install malware on thousands of machines, without any probable cause to believe that any specific machine owner was involved with child porn. If I understand the law correctly, they would need to obtain a specific warrant for each machine they wanted to search.

Let's say that there was a store in a neighborhood that was known to sell child porn. No judge would sign a warrant that gave police permission to put a GPS device on every car in that neighborhood to track whether they ever visited that store (and they may have visited but bought only legal merchandise). So why is it different if you do it on the internet?

That's a pretty poor analogy.

A better one would be 'Would a judge give a warrant to allow the FBI to place a GPS tracker automatically on everyone who visited a store that was known to sell child porn?'

The answer to that might be yes.

I think my analogy is more accurate. There were many independently owned web sites (stores) hosted at Freedom Hosting (neighborhood), and not all of them carried child porn. You could visit one of the legal sites without even knowing that there was child porn being hosted by Freedom Hosting (or even knowing where the site was being hosted).

From the article:

Freedom Hosting was a provider of turnkey “Tor hidden service” sites — special sites, with addresses ending in .onion, that hide their geographic location behind layers of routing, and can be reached only over the Tor anonymity network. Tor hidden services are used by sites that need to evade surveillance or protect users’ privacy to an extraordinary degree – including human rights groups and journalists. But they also appeal to serious criminal elements, child-pornography traders among them.