The U.S. has massive evidence of Iranian support for terrorist activity inside Iraq. This includes interrogations and captured equipment and documents. Iran just denies it all. That seems to work.
We show this evidence of Iranian help to enflame violence in Iraq and the Iranians just deny it flat out. Our politicians and pundits who don't think we should do anything more forceful than a UN resolution about Iran and instead argue we should just talk to them, work around the evident futility of talking to killers to stop their actions by denying that there is any evidence that Iran is doing anything wrong.
We have evidence that Iran is killing our troops and Iraqi civilians? Well, the Iranians deny it. Who are we to judge who is right? That seems to be their logic.
Or, as a fallback position, they assert that the evidence doesn't count because some entity within the country is really responsible for the action and not the government as a whole is. (I believe that's why the Congress let President Reagan off the hook for responsiblity for the Arms for Hostages deal. They didn't? Oh never mind.)
Like many other accusations of the anti-war side about the administration and war supporters, those on the Left who cry that intelligence has been slanted to support preconceived notions or existing objectives need to look in the mirror.
People are certainly free to argue that we should do nothing about Iranian support for terrorism in Iraq and elsewhere, do nothing about Iranian nuclear efforts, and ignore the horrible human rights record of Iran, but let's not pretend Iran is yet another kite-flying paradise unjustly accused of wrongdoing by the dreaded Neo-Cons.