We want people to really see everything before they buy. Self-interest wise its just easier to manage expectations when the buyer can fully inspect the product before any commitment. Anyone who can't afford it or would abuse it, is someone we cannot stop anyway and wont be happy paying for our wares.
I use the same psychology when I buy Cepheus Engine products - Buy and even Pay for the "pay what you want" ones. Of course its more difficult to manage expectations if I cannot inspect the product.
As for the FUll Preview I don't know why it reports an error