First you assuming this assuredly happened - Secondly Saddam was in violation of his missile technology - He developed missiles that went much further than the range allowed in rules given at the end of the First Gulf War (which the UN was enforcing....or trying to). -
To me the UN is a joke (for the record).
However, there are many reasons Saddam would have been paying off people within the UN outside of him having an active WMD program. The UN was monitoring a list of requirements outside of just WMD's -