People who floss twice a day tend to have all sorts of good habits. In fact, they tend to be the sort of people who just plain "take care of themselves."
Agreed, the cause and effect between heart disease and periodontal disease is not well proven. However, the beneficial effects of flossing in preventing periodontal disease is well understood and you're welcome to read the literature yourself if you don't believe me. I believe this is exactly what I said in my original comment.