Because they know that if they do anything, Jesse Jackson and Al Sharpton will decend upon St. Louis and bring their rent-a-mob along to insure the cops get at least twice the sentence this child rapist-murderer will get.
For the life of me, I have no idea why the police put up with all the hate and lawsuits and false charges against them. The day is going to come when some activist, some court, goes way too far in blaming the cop for doing his job, and the next day half the country's going to wake up to discover Blue Flu has spread across the country overnight. Let's see how well the left-wing haters like it when their calls to 911 result in the response, "Sorry, no cops on duty today!" All of us, of course, will have our own protection....
Oh please, this guy is white, Jesse and Al don't give a damn if a white person is harmed, as a matter of fact, they relish at the thought.