Exactly this. I did (and still do to some extent) domestic and small business repairs for nearly 20 years. Did I go snooping on people's data? No. I don't care what they do, it's none of my business.
My business was to get their machines up and running as well and as quickly as possible, and for an awful lot of people I was the only person they trusted to do that. That meant I was satisfied that the machine was running properly before it went back to the customer. I would almost certainly have booted a machine and seen it get safety to the desktop after having replaced anything, including a battery, even though it wasn't strictly necessary.
If the machine was seriously misbehaving, I might well have imagined the HDD, too, just as a precaution. If the image wasn't needed, it was overwritten within hours by the next machine's image, but on occasion that image was a lifesaver. Did I snoop on the files on the image? No, it's none of my business.
All that said, the images some people use as their desktop wallpaper are very NSFW, and you can't really avoid seeing rather more of the customer than you would like. Restoring to life a non-booting PC only to find your customer baring all on the desktop is an eye-opener, especially when they're sat right next to you.
There are certainly rogues in our business - I've undone the havoc created by enough of them to know that. But we're not all like that. Some (quite a few I suspect) take pride in our work, respect our customers and go the extra mile to get the job done right and to solve the customer's actual problem.