In this case, it's a legal requirement on the BBC. Their funding for iPlayer comes directly from the license fee that UK live-TV watchers legally have to pay annually. They do re-sell their content around the world and even have a commercial branch, BBC Worldwide, which creates content for the US and other markets, and also allows them to show adverts in those markets (which they can't do in the UK, again due to the license fee).

I guess they don't put everything on BBC Worldwide due to costs (getting permission from rights holders etc.) and instead prioritise those things they think are likely to sell.