Why should buzzing a crowd be okay, but buzzing a crowd FOR PROFIT be illegal?
That's an easy one. In many fields, it is considered onerous to regulate hobbyists because the cost of registration and testing would be prohibitive. If you're making a living doing something, if you can't make enough money to cover the costs of proper training, regulation and insurance, then it's not a viable business.
Obligatory car analogy: in most places, it's legal to give your friends a lift in your car on a normal driving license, but you'll need a taxi license to take paying passengers. This, I hope you'll agree, is fair, as it aims to protect the public without encroaching on personal leisure-time liberties.
Another analogy: public parks. Many cities are now introducing bye-laws/ordnances to stop trainers running profit-making bootcamps etc without a permit. This is because these commercial users are typically much heavier users than members of the public, and therefore use up more of a limited resource, increasing grass damage and ground compaction. Even where the space isn't otherwise needed, there is still the issue of their activity and noise affecting nearby park users. Airways are also public space, and it's fair to the owners (everyone) that those extracting commercial value are controlled and perhaps even pay back in.
Note, though, that this ruling isn't about the personal use vs commercial use argument -- the judge's decision was that the regulations for UAVs are not directly relevant to model aircraft (differences in size and flight range are not inconsiderable between commercial drones and hobbyist vehicles). As such, expect to see the FAA bring in new regulations for model aircraft, categorising model aircraft by top speed, effective range, fuselage size etc.
Note also that in most countries that do have specific provisions for model aircraft, Pirker would still have been breach of the law due to the proximity to the helipad. Not only that, but FPV flying (Pirker flies with a camera and head-mounted display) is technically illegal in most jurisdictions, because the designation of model aircraft mandates that the pilot must maintain direct line-of-sight with the aircraft at all times. The need to maintain line-of-sight kept issues of range as effectively redundant in the designation of model aircraft.