I rented from Hertz in MSN last summer. While I didn't specifically look at National's lot, the rental parking is in the ground floor of the garage across from baggage claim, and it's open access without a gate to the overflow parking for all if the companies in a parking lot adjacent to the garage, which is then open to the main road without a gate.

So without any gate, they can't leave keys in cars, and without that, they can't have an Emerald Aisle.