Because the zone determines the activities you can perform in the area. For example, in a no-standing zone, you can only stop temporarily to drop off or pick up passengers and must leave the zone immediately. While in a no-parking area, you are permitted to drop off people and property.
Why do Americans call it a parking lot? In the USA and Canada the word lot can mean an area of land set aside for some purpose, in this case for the parking of vehicles.