Well, I don't really know the answer, but I'll tell you my reasoning. I agree that you are getting a free item without any other purchase requirement. I would consider if they sent you a sample directly to be a FREE item. But since you had to take this coupon to the place to get the free item, I called it a COUPON.
Thank you for posting this, Solstice. I thought there had been a thread about this topic before, but couldn't find it today. Seems that I have remembered it correctly.