This is exactly why you will see a patrol car parked at the bottom of a steep hill with a radar gun for hours at a time, but call them for a real crime and it takes an eternity for them to show up. My daughter was robbed at gunpoint a few years ago with two of her friends. They drove to a nearby friend's house right near a major PA Turnpike exit and it took 90 minutes for the officer to show up to take the report.
Of course, I was over there by then and I was livid. I asked why they couldn't use my daughter's cell phone in the purse which was stolen to triangulate a location and possibly catch the thief. I was told it cost too much money and law enforcement resources. Seriously. Catching an armed robber just isn't a priority. But catching citizens going over 30 mph when they descend a steep hill is.
Sadly, you are right. They would rather set up speed traps than do other police work.
Also, in the situation you described, it was a robbery in which nobody was physicially injured, is that correct? It’s possible that such are given low priority by police. I certainly hope a 911 call about a crime in progress would pull the cops from the speed trap job and respond to such a call.