Apple famously have total control over their vertically integrated platform. It's called a 'walled garden' for a reason. They could trivially do this because a

Apple famously have total control over their vertically integrated platform. It's called a 'walled garden' for a reason. They could trivially do this because as it is, they require that all apps are signed. Irrespective of the distribution method. To sign an app, currently, you need a contract with Apple. They just need a one line clause stating that you can distribute on other platforms if the app also appears in their store.