I also don't really get it.
I'm sure that limiting the demo to pre-orders helped boost pre-order sales, but it isn't helping sell the game to those on the fence. How can it be a good marketing strategy to market a game (through a demo) to those who've already committed to buying it (through a pre-order)?!
Sell pre-orders with other bonuses (collectibles, cheap DLC, whatever). Leave demos and betas to the public!
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