There's a theory going around that Amazon are aware that they might be seen as selling 'thin air' in their Kindle store, which is something you and I both know isn't true. Art isn't worthless; if someone has created something new using their talents (learned, developed or nature-given) then it has a financial value. This fact is the reason so many musicians get tense if you ask them why they don't give their music away for free. One does not need to hold something weighty and physical in order to have made a valid purchase. If my mechanic fixes my car without needing parts or the purchase of tools I still pay him. I don't know when we became so convinced that we need to be given something tangible in order to feel like a satisfactory financial transaction has taken place.
Amazon do not allow publishers and authors to set very low prices on their Kindle books, this is probably because they are attempting to maintain a perceived value on Kindle books. As soon as something is given away for free it is considered worthless, and 'cheap' is just one step away from free.
Some Kindle books do seem a little expensive but the larger the publishing house is the more mouths they have to feed, which is why the Kindle Marketplace appears to be so beneficial to small publishers. It's not in Amazon's interests for our work to become worthless.