That's an unfortunate side-effect of the Mac app store. It's too bad, because functional demos was one of the nice things about the Mac software ecosystem.
I hope demos are a feature they're working on.
There's Sparrow Lite, an ad-supported free version that's set to release in the Mac App Store but hasn't been approved yet.
Give it a few days, that was literally only released last night, and they're still waiting for the ad supported version to be approved. Up until then it was in open beta.