Seems to me they're charging for value-add services. From the page you linked: "Building your app is entirely free until you publish it. You do not need to cho

Seems to me they're charging for value-add services. From the page you linked: "Building your app is entirely free until you publish it. You do not need to choose your pricing plan until you get ready to publish your app." And that seems only to apply if you publish through them.