Why do plow trucks leave a heap at my driveway edge?

Have you ever wondered why the city plow truck leave a huge mound of snow at the edge of your driveway? Usually you just get done shoveling or snowblowing and you hear the rumble of a very large truck with all its flashing glory tearing down the street with a mission...to plow the road and leave a heap at the end of my driveway. I'm not saying it is intentional or anything, but you would think that by now someone somewhere would have developed an attachment by which they could help you with your driveway, not make it worse. Well I cleaned up the heap with my little shovel and no harm done. The city folks are very dedicated to their job and I don't know too many folks that could hop in to a huge front-end loader and drive it in a straight line at 2 am. I usually go for my run around 4:30 am and they are out in force. Usually half of the plowing is done before I get up. They do a good job, except for of course my heap!