the developers seem not to understand the issues in streaming and the resultant addressable market, or maybe they understand fully and are just trying to garner publicity with this stunt
much bbc content is only licensed for the UK. agreements with other broadcasters, rights holders etc. severely limit what can be distributed to other countries, even some bbc free to air content is not available on iplayer in UK for just these reasons, much 'bbc' content is either owned or restricted by other parties, even inclusion of stock photos in backdrops in 'bbc' programmes can limit distribution outside the UK
iplayer is already available on mass market set top boxes (youview for instance), and smart tv units for the UK market, that's almost the entire addressable market
no idea what % of UK households have apple tv but no other stb/stv able to access iplayer, but i'd bet it isn't a big number
unless/until the bbc can resolve how to gain rights to stream globally and collect the revenue to cover associated licensing costs, which i suspect will be around the time hell freezes over, it'd be a very poor use of resources to spend time/money on an app with such limited market