Won't disagree on that. I think if they would really try, they would find a way to do all those things on Apple TV. I had an app from a german broadcaster that also lacked subtitles support for a long time. But a while ago they finally added support. Although you can see that the subtitles don't respect my style settings. So I guess they are using some workaround. And I could imagine that subtitles support lacked because of the same reason.
But on the other hand I think it's also true that Apple is kind of ignorant sometimes in some cases. And thinks they can afford to do things differently than everyone else. And regarding phones and tablets they can. Because they have a leading position there. But not when it comes to streaming boxes.
But obviously there is broad support for MPEG-DASH on other devices. Because otherwise the app would have the same limitations there too. But I’m pretty sure that those devices will also support HLS. So you could also argue that if there are two very popular protocols out there, you should support both. And that you can’t expect from the BBC to change their protocol just because of that one streaming device out there.
But I don’t want to take a side on this. In the end it’s just the usual blame game of big companies. And the customer has to pay the price for it.