Why does the headline state "Study Links Diet Soft Drinks With Cardiac Risk" when the story says otherwise?

Good point, but the title does just say soda is "linked" to cardiac risk. Based on the study that was done, the title is accurate although not the whole truth (and therefore misleading). They were studying effects of soda and determined that folks who drink soda probably have less than ideal diets and are more likely to have cardiac risk. That's a link. Plus the headline is supposed to grab your attention (which it did) so the writer did the right thing.