My only guess is Wasington wants to keep good relations with Russia considering all their stockpiles of nukes that they could be convinced to sell to radical Islamists.
Russia could have done it because of cold war treaties between old USSR/KGB etc. and Saddam. Also lots of 'oil for food' money.
When we stood up to Communism we had to make lots of dirty shady deals with a lot of dirty shady individuals.
Many of these individuals and groups are todays enemy in the War on Terror.
I wonder how many dirty shade individuals who we are using or need today as allies will be tomorrows enemy, in the next war after this.