You’re right. It’s foolish to think GS won’t look through all of your stuff (and maybe even copy what they want). But as one who has worked on PCs for decades I find that there is almost never a good reason to look though the contents of somebody’s hard drive when repairing the computer. Even if the problem exists in the drive itself there’s usually no need to look at their personal data.
A customer’s hard drive is like their garage and the tech has no need to know what’s in it, just to figure out why the customer can’t open the door and get to his stuff…
As for Geek Squad, if I couldn’t fix my computer myself I would junk the thing before I took it to them. I used to have a PC repair business and I would guess that about 3/4 of my customers took their machine to them first before they called me and told me GS only made things worse.