This feature has been introduced to make stolen phones useless to those that have stolen them.
However it can also arise when the user has changed their Apple ID details with Apple and not made the same changes to their iCloud account/Find My Phone on their device before upgrading to iOS 7, or if you restore from a previous back up made before you changed your details.
The only solution is to change your Apple ID back to its previous state with Apple at My Apple ID, verify the changes, enter the password as requested on your device and then turn off "find my phone".
You should then change your Apple ID back to its current state, verify it once again, delete the iCloud account from your device and then log back in using your current Apple ID. Finally, turn "find my phone" back on once again.
This article provides more information about Activation Lock.