It's called a traffic break. He's holding traffic back so that another officer up the road can safely walk out onto the roadway and clear debris, or get a disabled car out of the lanes of travel.
You're reading way too much into a cop car parked on a public street. FPS is the branch of Homeland Security that is responsible for providing law enforcement and security to federal facilities. The FPS officer driving that Tahoe could have been doing anything from running an errand to conducting...
Hey moron, on duty peace officers are exempt from a number of vehicle code sections, INCLUDING wearing a seat bealt and using a cell phone.