Why we resist direct talks with North Korea and Iran

Why does the US resist direct talks with Iran and North Korea? This may be part of it:
The Bush Doctrine had two basic tenets: Preemptive Force and Regime Change. The second has gotten relatively little notice, compared with the first. The idea behind regime change is that when a country poses a serious threat to us, say WMDs, getting them to back off of the threat, agreeing to a monitored end to the threatening policy or program, is simply not enough. They will hide their weapons, endlessly string us along, beguile our less vigilant allies. The only real end to the specific threat is to overthrow the government entirely.
I’m not sure exactly why, but the Bush government certainly appears to associate direct talks with settling for a policy change, and forgoing regime change. They fear that in direct talks they may get an offer too complete and comprehensive to refuse, and one which would certainly involve a simple promise of non-aggression from us.
North Korea has loudly made a non-aggression pledge a central issue, saying that without it they need the WMDs for self-protection. Iran has at least implied as much.
The key concession made by the US in ending the Cuban Missile Crisis was our promise not to launch a second invasion (the first being the Bay of Pigs). Some on the right still rue that commitment.