Because it seems that for study done there is another study about a year later that completely disputes it.
Coffee. Wine. Eggs. Butter.
Some of the prime things that make us happy went through this cycle, where they tell you it is bad, then it might be good, and finally, it is...okay, and then you never hear about it again until some douchebag starts up the cycle. Remember how they told us how terrible eggs were for you? Coffee? Wine? Butter?
Sensible people just partake, and try to reasonably do so.