Be honest and accurate with your descriptions?
I think the problem is not with users expectation but with developers. Web Apps are applications that reside on the Internet, that doesn't mean that it should just link to a website (although google does suggest that is an option).
Imagine downloading an iPhone/Android app that simply opens a browser window and links to the website. What's the point in that app? I can bookmark things myself.
If you can't offer anything new - like at least handling authentication for your service - then why make the app?
The Webstore should be more than a marketing tool.