My guess as to why the did not hire a truck as suggested above - is that they could see this free route worked.
They might have done some dummy runs with simple items and then used the route for profit. It could have continued for a long time and there are doubtless many similar operations running in the USA.
In Africa, it was/is not unknown for an extra (innocuous) package or suitcase to be added to the cargo hold and after departure, one ground handler rings his friend at the destination and tells him that it's on it's way. Taking advantage of opportunity is what human beings do.