You have to consider the uses these stock security levels are intended for. Higher security is for increasingly public use, such as web servers where space is let to the public. If you were the administrator of a public shared server you wouldn't want your customers to be able to log on and reboot it or shut it down, thus disrupting other customers' sites. You would want to reserve that power to yourself. That is what the setting you selected does. It is not intended for a desktop PC, but for a commercial web server, so it makes a greater distinction between what users and the administrator (root) are allowed to do.
Web servers are rarely shut down. Most desktops would be turned off when not in use, and therefore need to be easier to shut down.
Horses for courses.