I work at one of these third party locations. We ask everyone for their passcodes so we can properly test all the features of the phone. 95% of people have no issue giving us their passcode, and the other 5% just change it to something else before they leave it.
I guess a few of them might change it after the repair, but I can tell you most people just dont care.
And if it wasnt obvious, we dont share or sell any personal info or passcodes