The key thing here is that the operator/uploader is different from the app developer. As an app developer, you don't need to build or operate any backend at all, and you have no costs.
Buying local storage and paying for an external service both cost money, and the costs scale as your app gets more users. There's also maintenance overhead, you can't just deploy your app and completely forget those things exist. As a Skynet dev, you have no bills, no maintenance overhead, and it's truly a deploy-and-forget sort of thing. As the dev, you don't need any sort of Skynet account or signup.
Buying local storage and paying for an external service both cost money, and the costs scale as your app gets more users. There's also maintenance overhead, you can't just deploy your app and completely forget those things exist. As a Skynet dev, you have no bills, no maintenance overhead, and it's truly a deploy-and-forget sort of thing. As the dev, you don't need any sort of Skynet account or signup.