As to why she had a car, cars are cheap and insurance is cheap as well on Okinawa. The speed limits are low and there are not many accident and very few road deaths. As an E-5 she rated a car if she wanted one. She may have been married with kids, I don't know.
I will say that it is nice having a car on Okinawa. It is a small island but it seems bigger than what it is. There are soo many neat little nook and crannies to explore on the Island and you are not going to find them taking a bus.
I liked Okinawa better than Hawaii. The people are pretty friendly and helpful and the crime rate is low on Okinawa. They have some beautiful parks and the public play grounds are cool, the best I have ever seen.