The reason is simple, keep the best for yourself, everything we sell to our allies is still superior to most of the weaponry in service today, just not the cutting edge stuff. Why should we share everything...? We spent all the money developing the weapons/iPods/Software (you pick the product) why should everyone get off easy just buying a product....sorry, the world doesn't work that way!
Ty posted some stuff about how the US supplied various countries with weapons, not sure where you are from (don't know where noizyland is) but most countries sell weapons and other stuff to other countries, be it Iran, Iraq, Somalia, what have you....the countries that don't sell stuff just don't have anything to sell (at least nothing anyone wants).
I don't have a problem if the US keeps the "good stuff" for themselves, or if the UK, Australia, Iran, etc. do the same, it is their right if they were able to invent the thing...guess what folks, life isn't fair, if you suck and can't develop something for yourself then you don't deserve to have/use the product until the inventor sees fit...if ever.
Oh, TY the US is not based on fear other than the fear that everyone wants to mooch off them for aid, weapons, etc...take a look, the US helps more countries in the world than any other...not that the US is the best country in the world but we are at least trying to do something (right or wrong) rather than sit on the sidelines bitching about everyone else (many countries in this bucket).