"Pretend that President Obama going to a prison and kibitzing with prisoners is a good use of his time and good imagery. (But if he were to visit a police department to deliver a morale boost, it would rile Democrats everywhere.)"

At the prison, Obama was protected from the prisoners by bars. At the police station, COPs have guns, and you how he feels about guns. Not saying he would be in danger, but maybe someone would recognize him from an old wanted poster from his college drug dazed days.