Well that is what Comic-Con is for these days. People spend a lot of money to go internationally. Also Hollywood people that are known to spend lots of money on things they like. Be it unfair to the British that pay their fee, but you all pay our fee regardless if Doctor Who is on TV or not. And given that was for The Day of the Doctor (what became an international released movie that broke records), it would make sense to show that to the larger international audiance. That the BBC didn't also show you is the BBC's fault.
For the present, when they set precident, so they run with it because it is expected. And it draws in a lot of money. Money they wouldn't get otherwise, while the Fee they will get anyway from the British people via the BBC.