Bob Sutton nails it in his blog post:
HR people often get more abuse than they deserve because they are so often put in a position where no one notices them when they do something right, but they get blamed out of proportion when things go wrong. A classic fate of people who have responsibility, but not enough authority. So they often are very timid about taking action because they get punished for doing even the most obviously right things. Not all companies create this difficult situation for HR heads, but too many do.