> Was my mattress so cheaply-made that the manufacturer could afford to give them away?

Shipping is expensive, especially for something as heavy and bulky as an uncompressed foam mattress. Among other things, I have a free stroller (sans cup holder) and a life-time supply of Feather razor blades (they accidentally sent the bulk packaging) because Amazon didn't want to pay for return shipping.

Factor in the risks and headaches of trying to resell a used mattress, and I can easily see never wanting to take one back even if it means having to factor into the regular price the costs of finicky buyers. The mattress market is weird: http://freakonomics.com/podcast/mattress-store-bubble/