Not bizarrely at all. Same rule kept Harris out of competitive games and stops many players going to Europe before 18.
FIFA's attempt at stopping child exploitation. Too many kids have been picked out of villages by European clubs, trialed for days/weeks/months and then turfed out on the streets of Paris* with nary a penny.
*or other city, not picking on French clubs...
The LOTG seem pretty clear:
1. International transfers of players are only permitted if the player is over the age of 18.
2. The following three exceptions to this rule apply:
a) The player's parents move to the country in which the new club is located for reasons not linked to football....(unlikely applicable in this instance ?)
b) The transfer takes place within the territory of the European Union (EU) etc .........(not applicable in this instance)
c) The player lives no further than 50km from a national border and the club with which the player wishes to be registered in the neighbouring association is also within 50km of that border etc.....(not applicable in this instance).
I reckon that in order to circumvent the above Law, the Nix don't "register" the (minor from overseas) player(s) therefore no official "transfer" has taken place & hence they cant play those un-registered players in competitive games .
(If they did "register/transfer" the players (including Harris when he moved to NZ?) it would seem likely they are in breach of the above LOTG?)