Marketing is marketing. Trying to sell a product that's yet to be finished can lead to this situations, especially when combined with publisher's need to hype everything up. Considering how big preorders are and how fast the value of digital content decreases, there's no real reason for publishers to stop this. Hell, sometimes it's inevitable because the game is not finished and they are trying to sell you a product way before completion, so they have to show you something. With that said, they are still at fault for trying to sell the pelt before hunting the bear. Best advice is always to wait, there are thousands of games out there, no need to preorder anything anymore (unless they are limited retail releases, and even then those are niche titles that rarely promise the world with their marketing). Trailers can promise you the world, but if you wait a month to buy the game, you'll see if they are telling the truth.