The assumption was that 80% of defendants are guilty, which is more than 4 of 8. Under this assumption, asking whether p(guilty|convicted) > 80% is just asking whether conviction positively correlates with guilt. Asking if p(innocent|acquitted) > 20% is just asking if acquittal positively correlates with innocence. These are really the same question, because P correlates with Q iff ¬P correlates with ¬Q.