Their website has a funny disclaimer about the buy-on-board. I was looking at the SHV
so this might just apply when looking at LAS
flights. It says, and this is not a quote but rather my memory "we are the cheapest airline in the industry. Let the big casinos bring you the free drinks. If you want one in flight, you'll have to pay for it" And that includes paying for soft drinks.
They also sell Elvis sunglasses, neon Las Vegas necklaces, etc.