Ameriflight and it's competitors fly very time-sensitive freight; checks, UPS/Fedex/DHL, donor organs, etc. It can be somewhat surprising what people will pay to ship by air; my company had a contract with the Wall Street Journal for a while; they were printed in Denver, and we flew them to Salt Lake City, Albuquerque, and Billings, MT. See the Strangest Freight thread for more.
Granted, that does not answer your initial question; I too am curious about that.