I'll try another dealer to see if they see suspension issues (I'm sure they will since both the dealer and the tire dealer did) and if so, see if they will do it under warranty. If they don't do it under warranty, I'll call Kia.
I have been having my tires rotated, but probably not often enough. So, that can cause cupping too and it isn't necessarily the suspension? Hmm, are there other symptoms of suspension damage that the dealer can see, or are they likely assuming the trail arms needing replaced from the tires? If another Kia dealer says the repair is on me and not the warranty, maybe I'll get a new set of tires and see if they still diagnose suspension work. Other than the slight noise in the rear from the cupped tires, there are no obvious signs of suspension damage while driving.