I would believe that people who dont necessarily want to buy a new stinger may be starting to hunt for a used one. Demand for used stingers could be increasing. Surely they wouldn't want a car on their lot that they weren't sure they could sell for awhile.
I dont know how kia does these types of gimmicks, but VW would send us these on my Wife's tiguan about every 6 months. It was 5 years old and they were offering a decent price on it actually. Based on the letter it didn't seem contingent on us buying a new car from them but who knows because we weren't ready to get rid of it at the time anyways. Unfortunately when we were getting ready to graduate to the
Telluride, VW had lost interest in our Tiguan.
In any case no ones going to hold a gun to your head and make you buy a car. If whatever they are trying to pull is beneficial to you do it, if it stinks tell them to pound sand.