There's more than one way to do this. If the purchase is refunded on the device, the app can be uninstalled at the same time; this is how Google does it. If the app is refunded on another device or in the browser, then all it takes is for the app store to keep track of app licenses (which it already does) and remove apps when their license disappears.

It seems odd, though, that Apple wouldn't have an API that allows locking app functionally behind licenses; on Android [1],

> With Google Play Licensing, your application can query Google Play at run time to obtain the licensing status for the current user, then allow or disallow further use as appropriate. > ... > Note: The Google Play Licensing service is primarily intended for paid applications that wish to verify that the current user did in fact pay for the application on Google Play. However, any application (including free apps) may use the licensing service to initiate the download of an APK expansion file.

In other words, the licensing service checks for both IAPs as well as if you've paid for the app itself.

[1]: https://developer.android.com/google/play/licensing