my gut feeling is that the reason some countries are allowed to "swap" their license for a UK license, has to do with agreements between countries. It is quite possible, probable even, that the US will not allow UK license holders to get a US license without a full test. In wich case the UK is probably just returning the "favor".
Having a time limit on how long you can drive on your license, makes sense when you consider the rental and tourism trade. Not all Americans are comfortable with public transportation. My car got totaled a couple months ago here in the states and I've been on the local bus and train system in Atlanta. It sucks compared to England, but I'm getting used to it. Most of my friends act as if I'm suffering the worst kind of cruelty. Most Americans who are able to travel overseas, just don't do public transportation.
well there are my thoughts on the subject.