In aviation, if you receive ANY form of compensation for flying, that flight falls into the category of a commercial operation. That's right, even if that compensation is only gas money, or a cold beverage.
It is part of the FAA's mandate to protect the public, and this is one of the ways that they can do that. When the regulations were written, they were protecting people IN the airplanes. It's no secret that regulating Drones is and will continue to be difficult within a framework of regulations built for traditional aviation. Hence, Part 107...which is honestly a great step forward.