From the FAA perspective, why is 'compensation' so tightly regulated and interpreted?
Because time and time again, people skirt the rules to hold themselves out commercially to do on demand charters, without following the rules.
Like.. buying a logbook for a non-flying customer and logging it as instruction given... while flying that customer all over the place as a part 91 charter pilot.
If you want to make money (or lose less than your fair share) off your friends, then you have to toe the line, because your less scrupulous brethren have ruined it for you.
The underlying premise is that the more people who can be endangered, the more regulation required.. Roughly
Part 91 and part 103
Ultralights (pilot only, weight limits,)
Sport pilot (pilot and one pax,)
Private pilot (more requirements but more privileges)
Part 135 On Demand Charter
FAA oversight, maintenance requirements
Individualized Ops Specs that serve as your company's federal regulations
Commercial pilot.. 100 hr inspections or progressive maintenance
Chief Pilot requirements
More paperwork
Airlines (Part 121 and others)
Above and beyond 135
Much greater FAA oversight, Ops Specs, More paperwork
ATP's