I'm proposing no such thing.  Treaties are simply agreements between actors within an anarchy, they have no direct force upon Osama or his fellow travelers.  They could have mined and refined their own U235, but they didn't.  Why?  Because it was beyond their expertise and resources.  I did not propose that those who can refine U235 (mostly major corporations in Western nations, and a few Western governments) sell Osama weapons grade materials.  But if they did, they would be accountable to their treaty peers (anarchist rules, remember?).  Any government agency or corporate entity would be liable for the results of selling finished products to customers that they do not know.  This is the very reason that fuel grade U235 isn't refined to greater than 20% by mass, as a rule (but not an absolute); because there are few reactor designs that require greater than that level of refinement while there are zero nuclear weapons designs that could even theoretically work at less than 50%, most require 95% or greater.  Thus, as I mentioned, weapons grade u235 has no established legitimate civil use; and persuit of it is thus a rational reason for society to be suspect without your explaination.  If a legitmate civil use of 50%+ U235 fuel assemblies were to be established, then you might have a point.  But that isn't the case now, so you don't.