Roger gave you the correct answer. Yes, technically they could claim it
was "compensation" and come after you. Should you have let that stop
you? No... I would have done it, and I suspect most all of us on this
list would have also - without a second thought.

You were just "helping out" and I would hate to live where we were all
so terrified of big brother that we were afraid to give a friend a lift
to pick up his plane, or help out some folks stuck away from home.
Should you worry about a letter from the FAA for this - absolutely not.

So where *does* the rare action come from? It usually comes from
repeated actions, where someone has a vested interest in stopping you
from doing it. One of the cases on the books is glider tow, where the
company operating the aircraft will exchange free flight time if one of
the pilots who is also power rated and checked out will work a couple of
hours of tow first. [Nothing wrong with a bunch of glider pilots simply
trading off who tows whom. This is a different situation.]

Here the private pilot is getting free flight time in exchange for
flying duties. Yes, it's done all the time, so why this action? Well,
it COULD have simply come to the attention of some FAA type hanging
around. But more likely a commercial tow pilot who couldn't get enough
work dropped a dime on the FBO.