There could be any number of reasons why she chose to walk to your door and not the others, it could well be because none of the others had vans with the engines running in front of them or there was nothing to draw her attention to them. We can't know.

Your dislike of the situation and disagreement with the current standard of the law aside, based on your description the officer did nothing outside of policy or the law and what little you CAN do, you did. You called the agency to voice your disapproval. You can make additional calls if you'd like, but it will likely do little to stop a common practice by law enforcement everywhere.

As I suggested, if you want to keep out unwanted visitors - including unwanted police officers, lock the door.