My guess is that the US government and Japanese government have agreed that members of the American military and their dependents can use vehicles which are not subject to the same taxation and inspection requirements as cars which are owned / rented / borrowed by Japanese ciitizens and most foreigners in Japan.

Probably... if you don't have US-military-type status in Japan, then you don't qualify for that exemption.

(Furthermore, your IDP will only allow you to drive for the first year you live here - after that, you need to get a Japanese license. I imagine that US military are exempt from that requirement too... ).

It's just my guess - if someone knows different, I'm sure they'll correct me.