Very interesting. I checked, and DSM does allow free unlimited miles on one-way rentals, such as the ever-popular one to Chicago for cheaper flights. If this were an independent franchise fleet, one-ways would either be prohibited or come with a separately itemized steep drop charge. The fact that one-ways are allowed at the usual pricing scheme of a high daily rate with free unlimited miles means this is the same randomly circulating nationwide fleet everyone else is using. So, this particular franchise operator may have implemented their own policies on roundtrips.
When I visited DSM, I did notice that almost all the rental cars had Illinois plates. Many of them apparently came one-way from Chicago, which is more expensive than the reverse trip. There were some registered in Iowa, and one or two registered in Nebraska and Missouri.