Yet it was the federal government that dispatched troops to curb the excesses of KKK-rightwing populism. It can be also argued that the Civil War was a federal effort to curb the excesses of racist populism.
The point I am trying to make is that while the US elite (by no means a unified entity) certainly benefits from low intensity social conflicts, it stands much to loose from a powerful populist movement. This is why it may let those low intensity conflicts simmer, or even instigate them a bit if they start going out, but at the same time keeps a tight lid on them to prevent them from becoming a mass movement.
This is playing with fire, because calls for pogroms and ethnic cleansing are undoubtedly the most powerful force mobilizing a mass movement ever. But the fact that while US is rife with deeply seated social conflicts, it has not been engulfed in a populist holocaust is the testimony how skilful the US elite is in manipulating popular sentiments.
Of course, it needs to be added that the US elite is so successful mainly because it stands on the shoulders of giants, especially British imperialism, which perfected the so called indirect rule. Indirect rule relied on simmering local conflicts and cooptation of some local powers to the colonial project - and it worked quite well for the Brits. They only started losing when they became too greedy and began to undercut the local elites that were instrumental in maintaining their colonial rule (cf. the Mau Mau rebellion).
Wojtek