Customers are disappointed because their expectations will not be met. When they purchased Sparrow, they expected to receive future upgrades, too. They expected this because that’s the norm for the App Store. But I don’t think it’s fair to say that, because that expectation was reasonable based on unofficial and unstated norms, the Sparrow team violated their trust by stopping development. If that were the case, how many major upgrades should be expected? One? Two? Three?
So much outrage after Sparrow was purchased by Google, and people felt cheated. I wrote a piece on this subject a year and a half ago and nothing has changed. The Mac App Store has completely changed people’s expectations for updates and future features, but I don’t know how people think developers will make money for future work.