This is by design (by ubuntu developers).
Numerous users have complained about this and filed bugs regarding it. But the Ubuntu developers have not been responsive to implementing this feature request. The situation is much worse for users who use dconf-editor to disable the unnecessary "are you sure" prompts that appear when users request Shutdown, Logoff, etc. When this is activated, there literally is no way to shutdown via GUI.
You only have two options (far as I know) to Restart the computer in 12.04. Use the cumbersome and inefficient "are you sure" prompt that occurs when you select Shutdown.... Or restart via Terminal (which involves having to type in your password everytime!) with the command
sudo shutdown -r 0