Why is it forbidden to nap in a car?

During my lunch break, I drove to the mall near my company and took a nap. I didn't recline the seat or anything. I just simply parked my car, turned the engine off, put my shades on, and closed my eyes. Less than five minutes into my nap, a security guard bangs on my window. He tells me to "wake up you idiot. Sleeping in the car is a big no-no! If I ever catch you sleeping in the car again, I will have you arrested. I could've had you arrested if you reclined the seat. Now either go inside or leave the premises. Gosh that is probably the stupidest act you can commit."