If Apps are delivering new content and functionality in response to users directly giving them money then in what way is the walled garden walled? Just download

If Apps are delivering new content and functionality in response to users directly giving them money then in what way is the walled garden walled? Just download the "door" app - now you're downloading arbitrary apps from a 3rd party. What happens when you download steam and suddenly people are downloading totally arbitrary code. Oh and that arbitrary code is hijacking whatsapp and sending itself to all your contacts?