BT provide the app to retain customers, the customer quite rightly expects anything provided by BT to work properly. A customer would also expect apps to have the same functionality as provided on other platforms, otherwise why would BT provide it? They can't be that naive surely.
I don't know if it's down to cost, a lack of testing, STB capability etc, probable all of these and more, but the app is an intrinsic part of the STB, not a physical add on, so the responsibilty for it's performance is with BT as far as the customer is concerned.