Because user didn't entered password and he can't change password because he doesn't know his current password.
Actually user can set up password if he registered via facebook. He should log off, then open /forgot-password and then he can set up the password. But I don't think that any of facebook user would set up password.
My idea is: not to show password change button until the password was actually set.
Anyway it's not a big deal but just unnecessary and odd button for the user.