Following the money to security
That's why Google Play should have a "financial model" tab in the Android app library. The developer should explain where the money is supposed to come from or why he is developing and offering this app, and Google should add a comment (that the developer cannot modify). It can be as vague or as specific as the developer wants to get, and Google doesn't have to say anything substantive about it. Here are some examples:
Developer: My revenue will come from the professional version of the app.
Google: For every 10 free versions, 3 professional versions have been downloaded.
Developer: My revenue will come from advertising.
Google: The developer is participating in our advertising program and has received revenue ranking him in the second quartile of advertising-based developers.
Developer: My application development is supported by Company X to support their hardware.
Google: We contacted Company X and did receive confirmation that this developer is receiving some funding, but no details.
Developer: I am independently wealthy and I'm creating this app out of the goodness of my heart.
Google: We have no evidence supporting this funding model.