Some BBC stuff is restricted to the UK because of limits placed on the BBC by the programme owner who is hoping to sell the same thing to other broadcasters. Obviously if you can watch it for free from the BBC it means it's less attractive to overseas broadcasters.
Some stuff will work with iPlayer, the radio stuff certainly does because I'm listening to it now. It's not actually that hard to bypass the restrictions if you have access to the right stuff and a bit of technical know-how.
Subject to the commercial restrictions, the BBC could probably make some money by offering a subscription service with a modified iPlayer for people outside their area but whether it would be worth the effort is another matter.