I assume in their case they were investing in the infrastructure as well, which would be a significant cost; if one just resold this service on top of Amazon fo

I assume in their case they were investing in the infrastructure as well, which would be a significant cost; if one just resold this service on top of Amazon for cost +x% they wouldn't have that kind of capital exposure, not to mention no marketing / advertising if it's kept as a small side project. The thing that I would worry more about when running a service like this would be liability for what users of your service do / say.