Because, I am not convinced that whoever is responsible for sectarian violence in Baghdad is entirely motivated by a desire to control the national government, rather that there is a more sinister form of hatred playing out on the municipal level. My fear is that after a withdrawal, the national government might be motivated to aid and abet the Shi'ite militias in Baghdad, and could transform a more inchoate form of sectarian civil war into something more efficient. But that's not the same as thinking that if Shi'ite leaders with connections to the militias controlled the government, their interest in using force against Sunnis in Baghdad would be satisfied. No, that opinion is not based on the kind of information or mastery of the facts I would want policy makers to have -- it's based on the volume of sectarian attacks in Baghdad, and the shift away from US forces as targets since about 2005. I reason from this evidence that those repsonsible for the violence are as motivated by mutual antipathy as by the US presence, and that involvement in sectarian violence in Baghdad is widespread and popularly based, rather than limited to a tiny minority of trained fighters. In short, the kind of thing that suggests a goal on the ground other than acquisition of the state.
Nicholas Beaudrot argues at the link that the risk could be reduced with timely negotiation. He is right that if this the concern, it is theoretically possible to bribe whatever group begins to take on state-like features during the civil war. I would want to know more about the Shi'ite leadership outside the government to know whether this is possible. Speaking more generally, there are a number of international regimes that opted for brutality at home rather than aid and trade. If anyone has any thoughts about why the whoever is likely to take power after US departure is not likely to be one of them, I am all ears.
In short, the reason that I believe that US forces are a net security asset is that I don't believe that most of the violence is motivated by a desire to get rid of us. Militias in Baghdad might well use it as a recruiting tool and then deploy their recruits against sectarian rivals. But given the frequency and volume of sectarian violence, my suspicion is that the critical mass necessary for a more orchestrated campaign in the city is already adequate.