I suspect in the state of NY there have been many who have been convicted of possessing child porn based solely on files found in temp caches.  Want to bet whether their convictions will be retroactively overturned or expunged? Their rights restored?  Removed from the sex offender lists?  Allowed to sue?

Since this was in the arrested's computer cache and not a file purposely saved by him, that means the outfit he hired to remove a virus infection deliberately viewed his caches. 

In removing viruses you would normally delete any temp caches, and maybe look at file names to figure out what virus it is...but viewing the actual files themselves, especially image files? 

Sounds more like they were deliberately looking for illegal activity as a routine activity on customer's hard drives. 

At the government's bidding?  For a reward?  Would you want to take your PC to those folks...even with nothing to hide...except maybe financial records?  Would you trust such PC techs with your bank account and credit card info?