Some of the time this will be an obligation based on content rights - e.g. the BBC only have bought the rights to show Premier League highlights or the Olympics in the UK, where as in the US NBC or ESPN might have bought those rights.
Other times, this will be due to the British government not wanting to be seen to be undercutting commercial providers in other countries (there's enough pressure on the government in how the BBC is run from commercial providers in the UK).
What's more frustrating is when the BBC produces content (e.g. via BBC Worldwide or BBC Foreign Service) which isn't available in the UK. Fortunately that's fairly rare.
I suspect it might go further than not be seen to want to undercut commercial providers (though in the case of this government that's absolutely true), I suspect that it might be illegal for them to do so - state subsidies and all.
Other times, this will be due to the British government not wanting to be seen to be undercutting commercial providers in other countries (there's enough pressure on the government in how the BBC is run from commercial providers in the UK).
What's more frustrating is when the BBC produces content (e.g. via BBC Worldwide or BBC Foreign Service) which isn't available in the UK. Fortunately that's fairly rare.