U.S. DENIES IRAN THE RIGHT TO PEACEFUL ENRICHMENT
Because such knowledge would be a deterrent against attack

"... the position of the U.S.... has been that Iran must be compelled to forfeit its right to enrich uranium, even for a purely peaceful nuclear program, since the technological knowledge that Iran is acquiring by enriching uranium could be diverted towards a military program in the future...

(However) Iran's goal is not the acquisition of a nuclear weapon, but the acquisition of the technological knowledge to assemble the components of a nuclear weapon without actually building one and without violating the nuclear non-proliferation treaty... this approach could give Iran much of the benefit of acquiring a nuclear weapon as a deterrent against Western attack without subjecting Iran to the negative political and economic consequences of openly violating the treaty.

(So) The U.S. goal has not been merely to prevent Iran from acquiring a nuclear weapon - the U.S. goal has been to prevent Iran from acquiring the deterrent benefits of getting close to acquiring a nuclear weapon..."

http://www.commondreams.org/view/2010/05/04-4