The dumbest argument or the simplest valid reason?
Nope, it's definitely the most asininely stupid and ill thought out argument/idea I've seen on here in a long, long time!
Why should we do anything at all for Tonga?
You (i.e. the NZRL) aren't doing anything for them, so what are you talking about!
This Tonga situation will not last. They can't even produce 1 first grade standard half. This little period will go down as a flash in the pan. Circumstances worked in their favour massively last year, they will not be so lucky in the future. They won't become a tier 1 nation.
This game doesn't belong on our soil.
Firstly, the owners of Mt Smart, all applicable tourism businesses in Auckland, most certainly the local government and NZ government, the retail sector in Auckland, etc, etc, will all disagree with you on that point considering that they'll all see an increase in business/exposure thanks to this game...
Secondly, the fact that you (the NZRL) administer the sport in NZ doesn't mean that you own the sport or that you have an exclusive monopoly on the sport in NZ, and in the grand scheme of things nor would you want that to be the case as it'd end in tears for the sport in NZ...
Thirdly, you need to think this line of thinking through to it's logical conclusion, the conclusion where the sport effectively can't hold matches in neutral nations cause the local administrations will use the opportunity to extort the participating nations of cash thus destroying one of the sports best assets in drawing some attention in nations and/or communities that the sport is trying to grow into...
Also where was this 'concern' when the NZRL wanted to participate in matches in Aus and the US against England... I'm pretty sure that you would have been furious if the NRL and/or USARL were to demand some sort of compensation or were even to veto the match, but I'm sure that in your mind it'd be different if somebody else was to do it...