According to the news story not quite linked in the OP, the "test case" defendant was found guilty of two charges. One of those charges was "attempted possession" of child porn, based on clicking the link. It was not just the basis of probable cause for the warrant, it was, in itself a felony.
The second charge was based on two thumbnails stored deep in the bowels of his web browsers inner workings. They indicate that one site that he was on at some time linked to something with pictures that appeared to be naked children in suggestive poses. There is no indication that he ever visited the page with the thumbnails, much less that he saw them. And absolutely no indication that he clicked on the links to those pictures.
It is possible that if the FBI went over your hard drive with a fine-toothed comb, they would find evidence of child porn, too. Prosecutors love child porn cases, because it is so easy to find evidence, and so easy to convict on the slightest bit of evidence. For them it's not about getting real predators out of circulation, but about scoring political points for being tough on "pornographers."
They are doing a disservice to the public by doing this. There are real child pornographers and child molesters out there who are getting away with murder (sometimes literally) because the prosecutors are concentrating on easy cases.
