In my opinion...
This is the equivalent of inviting a plumber into your house to fix your sink and having him rummage through your dresser, finding your private porn stash, and making photocopies to take home. DUH! That should simply not be allowed!
If, however, that plumber were to see something in plain view that was illegal, like a dead body, while in the course of his plumbing duties, it would be his duty to report that to police. Thus, if while repairing a computer the technician happens to see something illegal, I think it's the same situation.
Once I had to do some work on a coworker's work PC and when I did I found it riddled with pornography. I wasn't being nosy either! I was embarrassed, shocked, and frankly humiliated! The coworker knew I would be working on his PC and didn't bother to hide anything. There were icons for porn programs on the desktop, porn wallpaper, you name it. I don't know why I wasted my breath on a gasp since he had a Sports Illustrated swimsuit calendar hanging up in his cubicle. Anyway, I REGRET not saying anything to his supervisor! He ended up leaving the company a few weeks later as rumors swirled that he was billing 65 hours a week but no one could figure out what he was doing. Unfortunately I KNOW what he was doing and it wasn't work!
But I digress! Snooping is human, but unprofessional. It should be illegal in cases like these - it's a privacy violation. If, however, it leads to catching criminals, I'm all for it!