Apple techs have access to external drives with multiple versions of OSX installed. If they need to boot your device to test the hardware itself, they will use one of these. If those tests indicate no hardware issue, their only next step is to check out your OS. That would require administrator access to your machine if you're using whole drive encryption.

I mean, I get what you're saying... they should have verified those things up front before asking for access, but I'm pretty sure they work on the concept of getting you the fastest service they can, balanced with the amount of customers they need to help simultaneously. My guess is that the admin password is a default question because they know they _generally_ will need it, so it's best get it up front rather than waiting hours or days for the customer to get back to them.

Personally, when I had to take my Macbook in, I just zero'd out the sensitive data, changed my admin pw to something temporary, and let them have it. I know this will be a TOTAL surprise, but the multibillion dollar corporation didn't use this as a chance to hack my life. What a novel thought.