Its a combination of profit and political influence which is what makes BBC and interesting animal. The BBC world service radio gets Britain a lot of global coverage and political mindspace. Showing sherlock homes free?? Maybe not.
This is a business decision. BBC executives have already (probably) thought about whether they want to charge now or make it free now (and charge later). Its the age old question every company and startup faces -- shall I increase my reputation now and charge later or should I charge now. As its been already pointed out, BBC already has a great reputation.
Different business models result in different pricing decisions. Maybe BBC has determined that keeping things free globally is just not smart business sense for it because of real revenue foregone.
I believe that BBC sells a lot of its programs to channels around the world. I also believe it also markets various DVDs of its programs. BBC entertainment is also available as a paid channel in many countries. If it made these programs free on the web globally that business might be in doubt.
BBC is currently in a binary model which is to be free in UK and then sell its programs globally. The unintended consequence of that it ends up denying global internet users its programs.
What would be best, I believe, would be to provide all choices. For people who are willing to pay outside the UK -- bring them into a subscriber model of some sort. This way BBC can continue selling its programs to companies/channels/customers around the world and let internet users watch BBC programs, for a small cost.
Who knows? BBC might actually be working on subscriber pays model on the internet. Here other considerations come into play: bureaucratic sloth in an organization as big as the BBC and the time it takes to role out a subscription oriented video website.