Why does Apple make you turn off your find my iphone prior to repair? as well as, how and why do they turn it back on find my iphone automatically before giving it back to me?

And what did you want to hear from us? If you feel your device was not handled correctly by Apple, go back to the store and ask for the manager. It could not have been on without signing in to your Apple ID which requires your password.

Turning off My My Phone is a published requirement by Apple.

Get your iPhone, iPad, or iPod touch ready for service - Apple Support