Companies might earn more by towing vehicles away but the time taken to tow one vehicle to a compund might be more productively (and gainfully) spent clamping a dozen other vehicles, so firms aren't always as keen to tow away vehicles as you might think.
In general, vehicles should only be towed away where they're actually causing an obstruction rather than just breaching parking regulations. In such cases, there is no requirement for the vehicle to be clamped first.