Because the US government is so well-known for abiding to treaties...especially with natives.

Second, while I think you make a good point about costs, you really didn't properly account for that plane. Assuming you get it for a mid-point of $20k and finance it at the longest term at a lower-end APR, and you put a little under half down, you're still looking at $300/mo roughly. Now assuming you go into town once a month, sure, you're saving money ($440 versus $600+) but if you go into town every-other month you might not be ($740 versus $600+).

Lets also consider plenty of people in these rural areas are near or below poverty. That means credit isn't easy to access if they can get it at all, so even if they get a loan for a plane it can be a higher APR, and then the payment is higher. Plenty of folks probably couldn't afford a $300/mo payment, and that's fairly unrealistically low.

Nevermind what Dingbat said about maintenance, storage, insurance, etc.