I believe this is not just an Apple decision. Where iPlayer is accessible is mostly down to the BBC. This article goes a long way in explaining the situation...
http://www.techradar.com/news/internet/bbc-global-iplayer-coming-to-iphone-and-a pple-tv-next-984443
It seems the BBC is treading very cautiously and personally, I can't blame them. They seem to be trying iplayer "there" and watching usage pattens etc before trying "over there". I think its all about controlling usage and making sure they don't oversubscribe themselves. I note that they have just allowed iPlayer on the Xbox360. Maybe Apple TV will be next.
I guess every time the iplayer gets easier and nicer to use, more folk will use it and the beeb wants to make sure they can deliver without problems.
Being a UK public (paid for) service, the beeb has a lot of pressure to provide "access for all" - if iplayer was difficult or problematic to use they could get into serious bother for excluding those of a less technical nature (not to mention the flack they'd get for "being rubbish"). They would rather not provide a service than for the service to be seen as lacking.
Thats my take on it anyway.
I hope we'll see iPlayer soon on the Apple tv - in the mean time I'll have to stick to using my iPhone, iPad, Xbox, Panasonic tv or iMac ( ! ).