It requires cloud servers and power which needs to be online 24/7 for the app to work. That isn't free. How do you think all this commerical IoT stuff works?
Sorry, it is very unreasonable. You bought the probably expensive machine. If the app is used to control it, the company better include the price for that in the product. And a subscription is only warranted if there are any server functions needed, but for a sous vide cooker, what in the world would it need to connect to their servers for?