While I certainly understand the frustration being felt (I purchased the app and was hoping for push notifications), I'm sure most people here would've accepted the offer. With all that's been written on how little developers are making on the App and Mac Stores, I highly doubt they were making enough to sustain a team of 5. I'm assuming they're an ambitious group and selling an email app would just have not provided them with the capital and resources to achieve what they want. So they perfect their app and then what? You've pretty much hit the end of that road.
Sparrow was up against the native OSX/iOS mail clients, the GMail native and web apps, with the App/Mac Stores as their main distribution channels, which make it nigh impossible to get your app any visibility save the smash hits.
All that considered, it makes perfect sense, to me, why they'd accept a Google offer.