If I remember correctly, there was a lawsuit a while back that may be related to this... a guy was leaving a company and asked his manager if he could use him as a reference, the manager said "ok" -- then proceeded to trash talk the guy to anyone who called.
The guy sued the manager, which I can understand, but he also sued the company... and if I remember correctly, he won. Which seems a bit unfair, but I reckon if I was being smeared by my former manager I wouldn't be thinking too clearly either.
At any rate, if a company can be SUED for something that one of their employees says of their own volition... then it makes for them to start forbidding the practice of giving professional references.