The problem wasn't really that the US wasn't sharing any WMD intelligence with the UN inspectors. It was that the intelligence they provided didn't turn anything up. As someone involved memorably told CBS, it was "garbage after garbage after garbage." Jacqueline Shire and Jeffrey Lewis also talk a little about it here, and you can get corroboration from the Senate Intelligence Committee's Phase 1 report, here, but the US was a key source of intelligence for UNMOVIC. The intelligence was just all terrible!
This is sometimes murky, so even folks who opposed the war at the time might sometimes find it hard to recall. But it was actually much worse than that the US seemed to be withholding information in a wholesale fashion. Once you realized that what they did have was junk, the case for war became even crappier than it already was.
Part of the murkiness, I should add, was because the Bush administration found it to be in their interest to downplay their cooperation with the UN inspectors after their leads turned out to be useless. This was perhaps the most bizarre turn: American officials used the UN's failure to turn anything up -- using, in large part, American intelligence -- as a basis for arguing that the UN's inspectors were incompetent.