As a developer of one of these apps (DokDok.com) I'd say commparing Google Apps Marketplace to Facebook apps isn't really fair. Data access is needed in order to provide real value to your existing Google Apps. The objective isn't to spread as fast as possible among your friend list for some sketchy reason.
That being said, I don't believe there's any official audit from Google so people with bad intentions could probably get their way for some time until the word gets out. An important point to keep in mind is that Google Apps APIs are accessed through OAuth (even email data). This means that as a 3rd party app, all queries on your data are made using a unique secret token that doesn't require knowledge of your password and, most importantly, can be traced back to my app.