It's a requirement imposed by lobbying of the UK media companies. Any content the BBC produces for its international service must be funded by private-sector sources such as advertisement or sponsorship, since the license fee is required to be used only for UK-targeted programming. But then such content cannot be shown within the UK, because that would be considered competition with the private-sector domestic media.
Yeah, after moving to the UK I was shocked and bemused to find out that the BBC in their arrogance saw fit to ensure that not only would I be denied some of their content whilst outside the country (as I had long grown accustomed to), but that they have taken appropriate measures so that those citizens within the country will also suffer at the hands of the all-powerful BBC administrators who no doubt have very grave reasons to ensure the sanctity of their content distribution lest someone put together all the pieces and obtain the whole picture.
God that makes me rage. It's even worse than a paywall as it's completely nonsensical.