Can anybody tell me exactly what this shell command does? ADB [Ice Box]
Google has noted here:


  A device owner is a specialized type of device administrator that has the additional ability to create and remove secondary users and to configure global settings on the device. [It can be used] to take fine-grain control of the configuration, security, and apps on managed devices. A device can have only one active device owner at a time.


(Emphasis mine)

So now you know why all other accounts have to be removed for Ice Box but not for other apps. You can add those accounts later, after making that app a device owner.

Also, if memory serves correct, once you set an app as a device owner I don't think you can remove that app. You would have to factory reset the device or use root access to wipe it out.