This is standard procedure at most comp repair shops. I was the lead tech at one for a decade. If a customer didn't want to give us their password, we had them create a dummy account so we could login and test our repairs. Any good repair shop goes through these steps, either by habit or by policy:
1. gather information
2. verify or diagnose problem
3. if necessary, authorize repair with customer
5. VERIFY complaint is resolved
6. return equipment to customer
Step 5 is very important. Surprisingly to some, our first job is not to fix what we consider broken. Our first responsibility is to resolve the reason the customer brought it in. It's an easy mistake to make to check in a machine, see an obvious problem, fix it, return it, and have the customer come back upset that we didn't fix the problem they checked it in for. This happens when steps are skipped above. One example of this is getting a computer checked in during a storm of recall checkins to fix a widespread issue. Techs can get in a rut and just plow through another recall and out the door without paying enough attention to it, only to get an angry call from a customer that checked it in for some OTHER reason and wasn't even aware of the recall, and their reason for checking it in wasn't addressed. They often don't give two hoots that we fixed something else, their main beef is we didn't fix what they asked us to fix. Sometimes they have a long drive or its otherwise very inconvenient to drop off and pick up, and this just winds them up more when they have to make a second trip.
I know in our case we considered a mistake like that to be totally our failure, and would at the very least allow the customer to bring it back in and get free rush service to fix the actual problem. The service manager usually paid extra close attention to it at that point, and would personally verify with the customer that the complaint was resolved when they picked it up. Often they were credited or totally refunded the original service charge also. Free service makes GM frown and tends to get techs yelled at later.
So cut them some slack when they ask for your password. If that bothers you, make a test account for them to use. They won't mind. Oh, and more OT, the geek squad ransacking people's computers... wow. At a loss for words. But, we LOVE the geek squad, they are a constant source of revenue for us. They attract business to our area, burn customers and drive them to us, and on rare occasion they even have to check in machines to us that they themselves have broken worse. (that's my favorite... I recall a wireless antenna cAX on a new just-out-of-the-box computer getting cut during a memory upgrade, as well as a computer getting checked in for no boot because they'd upgraded ram by installing a sodimm in a PCI slot...)