Geek Squad is one of the few computer repair/support companies that rose to nation-wide status and lasted very long. Any other company that I think of is actually just a handful of franchises. They didn't get to the top by inventing stories. You are much more likely to encounter crooks at a small unsuccessful no-name place.
Think of the profit motive: there's none for Geek Squad. That employee isn't on a commission, and adding a hard-drive sale isn't going to noticably impact even a single store's daily bottom line.
As far as qualifications -- I'm sure any GS person will be A+ certification, prob'ly plus a bit more. Computer techs at that level are educated at tradeskill/community-colleges or military.
On the one hand, there's prob'ly a reason they didn't go pre-med or MBA. On the other hand, they know a lot more than you do, and this isn't rocket-science, eh.
This kind of thing does happen, in all areas. You bring your car in for maintenance and they tell you the CV boots need replacing. You go to the dentist for a prophylaxis & checkup and they want to "up-sell" you on filling a cavity.
It's gonna happen cuz that's part of the experts' job, to test and evaluate.
If you think you know more than the expert, then clean your own teeth. But in this case, if that were true, you wouldn't have installed a lame trojan, eh.