When you press Control Alt Delete in a Terminal, your system reboots. This does not prompt for authentication to reboot. However, running sudo reboot does require authentication as a sudoer to execute. These commands appear to be doing the same thing. Why does one of them require superuser privileges, and the other does not?
Ctrl-Alt-Del combination only works when you're at the physical keyboard connected to the machine (as opposed to being connected to a remote machine via an SSH session). So it only works if you have physical access to the machine. In this case, asking for a password makes no sense - you can just use Reset button or even unplug the machine from the wall socket or pour a cup of coffee inside it - all those actions will effectively shut it down without asking for a password.
You can disable this feature if you want: see Console Security in Ubuntu wiki