Years ago in my native CO there were a couple of guides that got caught using small planes to run herds of elk off public land onto big ranches they had leased so their "clients" could hunt them. They also used the planes to disrupt the hunts of people on the public land. This caused the state to put laws on the books about using aircraft and other mechanized vehicles to harass wildlife. They also put laws on the books about the use of drones as soon as they became available.

http://www.fieldandstream.com/blogs/...g-and-scouting

So, if the OP has seen it in CO, then it is illegal and he should try and get the tail numbers off the aircraft and report it to the Game Wardens.