The term is used mainly because many policies in cadet forces command that the cadets and some organisations do not have enemies or unfriendly forces to fight them, and thus doctrine also says that on official documents or radio transmissions that they are referred to as 'unfriendly friendlies'.
Although this is enforced in a formal matter in many military cadet units, informally most cadets will call them 'Enemies' or some other name, many cadet units have their own in-joke terms for an unfriendly force, normally stemming from some joke about a fellow cadet member who is of a foreign nationality. Most of these terms are the names given to forces fighting or forces that had fought against that units nation, such as Vietcong, Al-Qaeda, Nazi and Imperial Japan, especially for countries as part of the Allies in the second world war.
The same goes for friendly forces, a squad or group of cadets that excel as a small group inside their unit or company at certain skills such as stealth and evasion, may be termed as a member of an allied elite force. For example, a squad of five cadets who are extremely competent at stealth tactics may be referred to as the SAS or Commandos, whilst a squad that (in the proper military) has many good marksmen may be coined as a Sniper squad whilst not being actual marksmen.
This varies from country to country, corps to corps, and battalion to battalion, many having their own terms for friendly, unfriendly and specialist squads.