Why do waste haulers consider their shipments a form of commerce?

Waste haulers consider their shipments a form of commerce because they are employing people and plant to provide a service at a price. They are no different from any other hauler. 
The distinction between the in-state and out-of-state wastes are likely to be challenged under the Commerce Clause.

Accumulating more land so that your country can continue expanding without running into problems such as overpopulation