I guess very few people here understand licensing of copyright licensing. If the BBC were seen to be offering copyrighted material, images, audio etc. to the entire world they would have to pay a big increase in the license fee to use that material. The best example I could find is in licensing visual Artists' works http://www.dacs.org.uk/licensing-works/price-lists/television-and-film The UK v Worldwide fees are such that the worldwide fees are around double the UK fees.
In at least paying lip service to blocking viewing outside of the UK they are saving themselves fees in licensing copyrighted material and potentially residuals. ( Though how much the BBC actual spend on this is a mystery as the annual report does not breakdown costs that far.)