Why is it illegal to sit in your car in a mall parking lot?

I accompanied my friend to our local shopping mall, and she parked her car near the entrance to Macy's. I decided to wait in the car since she knew exactly what she wanted to purchase. Ten minutes after she left and locked the car, a police officer knocks on the window. I open the door and before i can say anything, he angrily tells me what I'm doing is considered loitering. All I did was sit very still in the car doing absolutely nothing except stare into space. Nevertheless, he gave me a warning slip and harshly demanded I exit the car and go into the mall. I was and still am in shock. I don't even think I did anything wrong! =(