For a non-pro runner, I think it is the right thing to do, especially if you know that you had not shot at winning but for the wrong turn.
While it was a trail race, it takes minimal effort to make sure that a course is idiot proof. $5 worth of plastic yellow construction ribbon can make idiot proof barriers to make sure runners on a trail do not take a wrong turn. Runners are charged with knowing the course, but race organizers should also be charged with making sure that the only way a runner gets off course is if he jumps a barrier or runs through a bunch of cones and ignores a race marshal/police officer waving him in the opposite direction.
I have missed turns before, even on races that I had done a number of times. Each time it was in the home stretch when I was redlining it. That part of the race is where it is critical for organizers to make sure that all the runner have to think about is racing hard. The rules may charge a wrong turn to a runner, but the reality is that it is the organizer's fault for not paying attention to detail in the course preparation.