From documentation file you've linked:
Complete Protection (NSFileProtectionComplete): The class key is
protected with a key derived from the user passcode and the device
UID. Shortly after the user locks a device (10 seconds, if the Require
Password setting is Immediately), the decrypted class key is
discarded, rendering all data in this class inaccessible until the
user enters the passcode again.
So device should be unlocked if you want to access that files. That is how Apple made it.
Edit: Found in "Hacking and Securing iOS Applications":
Protection class keys are master encryption keys used to unlock files based on their access policy.
Protection classes are the encryption mechanism used to enforce the access policies of files.
Some files are so important that the operating system should be able to decrypt them only when the device’s user interface is unlocked. These files’ encryption keys are wrapped with a class key that is available only after the user has entered his passcode. When the device locks again, the key is wiped from memory, making the files unavailable again.
Protection class master keys are stored in an escrow known as a keybag. The keybag contains the encrypted protection class master keys, as well as other keys to system files on the device. The system keybag is encrypted using another encryption key named BAGI, which is also stored in the effaceable storage of the NAND. Whenever the user authenticates to meet a specific security protection policy, the encrypted keys in the keybag can be decrypted.
So keys for that files are transferred, but are encrypted. When you enter your passcode, OS decrypt that key and it can then be used to decrypt files protected with it.