To be fair, many computer issues have been caused by malware that infected computers visiting porn sites.
There could be a high incidence of kiddy porn users having computer problems that were caused by their illicit online activity.
But Geek Squaders going on a fishing expedition for the FBI and even collecting a bounty of some sort is way over the line beyond just reporting a crime that was inadvertently discovered in the course of normal business activity.
Consider the police or feds paying auto-repair shops to examine cars for contraband while under repair, and have the cops pull you over as soon as you pick up your car.
If you rent, your landlord has the key to your apartment and is often allowed by the rental contract to inspect the apartment. Can he plant bugs for the feds while there?
Anything that is improper for "law enforcement" to do directly, is improper when done by a paid third-party.
Once the FBI crosses the line and pays the geek squad, they are no longer independent. At that time, they are agents of the FBI. As such, without a warrant and without full disclosure to their customers that the drives will be searched, it is an illegal search.
Geek Squad exists for one reason.
To tell you the following:
“Duh....your computer is broken. But, you can buy a shiny NEW one here at Best Buy!”