I have no issue with Geek Squad employees going to the FBI or local PD if they find child pornography while doing their job. It's what I would do, and I think it should be legally required that they do so (which it is). That's not the worrying issue here. The problem with this case is that these Geek Squad employees were...
paid $500 for digging around in customers' computers and reporting his findings to the FBI
So here's the potential issue: I have less of an issue if this was a targeted case, and the FBI was bolstering a case against someone they already had reasonable suspicion was engaged in trafficking child pornography. However, if that's not the case, there's two big problems with it.
1) The FBI isn't paying people to simply report accidental findings. These appear to be intentional, deeper searches for illegal or illicit materials. 2) If you are the FBI, and you have a budget for this, at the end of the year you're going to go back and look at your budget. If no informant has found anything, you're probably going to reallocate that money into some other program. If you're a Best Buy employee, you probably realize this. So there is an indirect incentive to find child porn or other illegal activity on a laptop, because it will keep you getting paid down the line. Therefore, there's an indirect incentive to plant it if you find none.
Finally, there's the issue of privacy, which is less of a direct issue here simply because the other issues with this are so serious.
To be quite honest, if you bring anything to Geeksquad you might need to re-evulauate what you are doing because the people there (from m experience and trying to get hired there among other things) shouldn't be allowed to touch a computer, let alone try to fix one for the most part. So this really doesnt even surprise me.
The thing about computer repair for the general population is that they have data on the HDD or w/e that isn't secured in any way and the person fixing it can just "accidentally" look through the data which is a breach of privacy but as you said, for ordinary people they have very little options available to them. I personally wouldn't let anyone near my data let alone leave my whole system with them. The only way you can be sure your data is secure when your computer is being fixed is if you know somebody like us who can fix it and morally wouldn't look at the data. I've fixed countless computers that involves moving data to a new storage device and I am extra careful not to look at anything if I can help it.
I would like to think that in a perfect world, it'd be kind of an unspoken Code among power users - If you're working on someone else's stuff, don't look at their stuff. It's just something you don't do on principle.
But unfortunately, you get some with a little more knowledge than most, along with a bit more avarice.
You can get an awful lot of power over somebody if you can see the contents of their hard drive because people store their lives on them. I once recovered 20 gigabytes of photos from somebodies computer, just imagine what you could learn about somebody with 20 gigabytes of photos, scary just thinking about it. It was entirely possible for me to make a copy of that data for myself and they'd be none the wiser.