It is not, Private Pilots cannot receive compensation period. There are situations where a PP can receive compensation while flying. At one time I had only an A&P and a Private. If I was working as an A&P and performed work on aircraft then flight tested it, I can stay on the clock and be paid. If I need to take the company 150 and some tools to go fix an airplane, I can be paid. If the same boss says "I need you to come in on Saturday just to deliver an airplane" I could not be compensated and further it could even be seen by the Feds as being compensated in flight time regardless. There are several other situations where a PP can receive compensation though most folks, students, time builders. Etc. are rarely in a position to legally do so. In summary there are situations where a PP can receive compensation while they are flying, though not for the flying and definitely never for hire.