It is specifically blocked in the USA and other countries.
The BBC sells its television content to other countries show by show. Those shows then have their own licensing and royalty fees. In the US, for example, many of the shows are split between Public Television and BBC America. The rights are then owned by those stations, not by the BBC in the US. Neither BBC America nor Public Television, nor iTunes are willing to let you watch a UK show before they have purchased it, licensed it, and are showing it on their own station, where they make money from cable, commercials, and other revenue-related sources.
You can't stream any content from iPlayer outside the UK on your iPhone.
When shows are released, they will show up in places like iTunes; Hulu; etc. in the US, other apps in other countries.