I seem to recall that weapons-grade fissive material is harder to produce than it is to produce fuel for a nuclear reactor.
http://www.ccnr.org/plute.html
If you pay one guy per nuclear reactor to watch out for plutonium "harvest", you can pretty much keep an eye on the whole thing.
"The disadvantage of reactor-grade plutonium is not so much in the effectiveness of the nuclear weapons that can be made from it as in the increased complexity in designing, fabricating, and handling them."
^So "cheap" fissive material requires expertise and constant maintenance to make weapons from. Weapons-grade fissive material is hard to produce, therefore easier to keep track of.