The NE one was probably a one-way. When I visited Omaha, the locals had cars with MO plates parked in their driveways. The rental car agencies were mostly across the Missouri River in Council Bluffs, IA.
The hotel airport shuttle driver explained that Nebraska has a will tax on car registrations, which a lot of people try to escape. Missouri has relatively lenient requirements for registration and residency setup. The meandering flow of the Missouri River means the airport approach road crosses from NE to IA, then back to NE, so it coincidentally works in the rental companies' favor.