They're operating on a different set of axioms.
They presume "war is bad", "war is worse than any other option", "war must be opposed at all costs", and "don't harm innocents/bystanders/noncombatants". It's just their basic starting point for whatever reason.
Being unwilling to weigh the alternatives (the notion "give war a chance" is absolutely unthinkable), their overriding goal is to stop what's happening.
Interestingly...
They also hold the axioms "we support the troops" and "bad people must be stopped".
Combining these and the prior axioms results in cognitive dissonance, with such bent results as
- pursuing non-binding resolutions saying "stop, or we'll say 'stop' again"
- authorizing funds to pay for combat they verbosely oppose
- spending years calling for the removal of Saddam Hussein from power, yet objecting when someone actually does
- calling for wars, yet expressing shock when anyone (the most violent enemy combatants included) are harmed
- confusion at the notion that war is not "surgical".
That's true, of course, but the "war" already happened. In 2003. That ship has sailed. What the people I'm talking about are obsessed with isn't even "a war" per se, it's a military occupation and reconstruction. The people opposed to this are so blinded (for whatever reason) that they don't even know what they're protesting. The "war" they're so against (they're still arguing about "WMDs") already took place. To listen to 99% of these people, it's as if they think that there's still a full-on "war" going on which they think can be "stopped" by pulling out our troops. (Paradoxically, these same people go on and on about how Iraq is in an unsolvable state of civil war.)
What I'm saying is, the simple belief that "war is bad" can't quite explain the obsession with the Iraq occupation.