Different situations, IMO. If the airline loses your bag, you are forced to shop right then and buy what is available not what you might otherwise select. If your bag is found, you have those additional clothes and the cost is covered due to the inconvenience factor. That inconvenience factor doesn't disappear if they never find your bag, it actually increases dramatically as you now have to shop to replace all your items.

Seems unfair to me (but not surprising )