As @codinghorror points out, all it takes is for an admin to download the backup and review any content.
If I had the same concerns about not wanting to let my users think their data was private from admins, I would add text to the footer at al times which stated who the admins were:
This site is maintained and supervised by XXXXXX
A constant but gentle reminder that someone is watching.
In other words, solve this concern with increased visibility, not increased limitations.