Exactly, nothing stops them to pull "Amazon", that is once they see your app is successful, they could request all data they need and either buy it off the comp

Exactly, nothing stops them to pull "Amazon", that is once they see your app is successful, they could request all data they need and either buy it off the company that developed it for you or commission their own app based on the documentation you had to provide and then block you. There is currently nothing you could do about it.