Nuclear weapons and material can be traced to its source region and sometimes even the mine based on the ratios of different isotopes. The US selling U1 materials to the russians means that anyone in the world with Nuclear material from U1 could plant a false flag that would implicate either the US or Russia. If the CIA gave material to a rogue state, it would be effectively untraceable since it would link back to the US.
U1 scandal may have been about much more than money.