You're not blocked for not being in the UK, you are blocked for being in the US. And the UK is blocked too (well, it depends on which BBC channel you are talking about).
Odd, I thought the entire UK was granted access, so long as they paid their "television tax" or whatever they call it now. How/Why does such additional blocking happen if that is not the case? Just curious.
BBC Worldwide is a subsidiary company of the BBC. It makes money from selling broadcasting rights, ads to other countries. This is fine because the UK public never sees those ads. Unfortunately, not being allowed to show ads to the UK public means they block access to all BBC Worldwide programs from within the UK. Some of those programs are entire new series eg (http://www.bbcamerica.com/orphan-black/) which the UK public has no right to be able to view.
If you attempt to access the totally free BBC Worldwide content within the UK you are presented the following:
"We're sorry but this site is not accessible from the UK as it is part of our international service and is not funded by the license fee. It is run commercially by BBC Worldwide, a wholly-owned subsidiary of the BBC, the profits made from it go back to BBC programme-makers to help fund great new BBC programmes. You can find out more about BBC Worldwide and its digital activities at www.bbcworldwide.com."
Its totally stupid. Everyone knows its stupid. But since almost everyone agrees the BBC is (surprisingly) good on the whole, any change to it is resisted on impulse.