Porters have a sightly different cam assembly up the front which varies fuel and blade angle scheduling. As I understand it they are approved for this operation, but I could be wrong.
I have always presumed that Beta in flight has never been formally approved in ag aircraft as it would present a liability issue one day when somebody (as agair3 pointed out) pulls in a heap of beta/reverse at the wrong moment, looses pitch and yaw control, and destroys the aircraft. Manufacturers have also never approved operations at the weights we are operating at, or approved diesel (of standard grade) for use in turbines.
Amongst a lot of reasons, one purpose for fast, slippery jet aircraft to chuck all those high drag devices out on approach is because they need a much higher power setting to overcome the drag. What does this mean? All the engines are half spooled up for the go around instead of sitting at idle.
Maybe thats why they invented FLIGHT idle. Doing 20 to 50+ landings a day exposes us to a much higher risk of needing to go around than the airlines. It's all about minimising risk at an affordable cost. However if the operators and insurance companies are fine with ground idle thats their choice.