Fairness and Justice in Grading

I have a policy on quizzes wherein if I make a mistake in grading that is in the student's favor and the student points it out, I do not take off the points that should have been deducted. On the one hand, this policy is not fair in that other students who made the same error, but were graded properly have lost points for something that this student did not. Both made the same error, but only one lost points. On the other hand, it seems to me that it is just in that it does not punish those who are honest. If two students both have misgraded quizzes and one points it out to me and one does not, it seems wrong to deduct points only the one who did the right thing.

Is there a problem with this policy? Should fairness or justice be the guiding principle here?