In OS X Yosemite, FileVault, "Enable Users" is not available

This is an inconsistency I've found compared to Apple docs on this, and to all the answers in forums online, and to another laptop in my care, similarly configured.

My user account is an Admin account.

Even after unlocking the preference pane for FileVault, "Enable Users" is not an option. Every help page or web search I've found has shown the "Enable Users" button in a screenshot of the prefs pane.

However, other users can still log in from the login screen!

Can anyone explain this behavior? I need to know what's up so I can administer my office's population of 20 or so Mac users.

Screenshot:

The Enable Users…-button is only available in the Preferences Pane Security & Privacy > FileVault if one or more users is/are not enabled to use FileVault.

You may test this yourself: open Terminal and enter:

sudo fdesetup list

to list all FileVault enabled users

Remove one user:

sudo fdesetup remove -user user_name

wait a few seconds and open System Preferences > Security & Privacy > FileVault.

Quit System Preferences > Security & Privacy > FileVault.

Add the user again:

sudo fdesetup add -usertoadd user_name

and enter some passwords/recovery keys.

Open System Preferences > Security & Privacy > FileVault et voilà: the button is gone.