There are countries which have very little or no real agriculture of their own, and survive (flourish, even) through exporting other commodities which they have in abundance, usually oil or precious metals/gemstones (or, once upon a time, salt). It's also hard for a nation (Rheinland) to have a war over territory with a group which doesn't actually own/control that territory (for all intents and purposes, control = ownership). Rheinland lost, and the treaty says they stay out. That's a treaty with the GMG, not Kusari, which means that Rheinland at least recognizes the GMG's sovereign control over the territory, however grudgingly.
The GMG situation is odd, but I think it's clear that they run Sigma-13, and don't rely on anyone else to do it. If necessary I imagine they could move their corporate holdings out of Kusari completely, but it's probably easier not to.
