Movements or fads like Avocado-based diets or the practice of drinking one glass of alcohol a day are really hard to justify scientifically because the human body has such a multitude of variables. It's almost impossible to test the effects of such a diet ceteris paribus. So I'm not sure if "science" can really be blamed. The general public wants a simple and easy solution, so the "science headlines" are going to try to give such a solution with half-proven theories and loosely-correlated results.
Not just a variable thing, also a time scale thing.
I put my hand in a fire, I get a burn right away. Pretty easy to know what happened. Sure there were many variables. My shoes, my pants, the weather.. but pretty clear.
Drinking a drink a day may do something good. May do something bad. But both those events happen in 20 years. Holy crap that is hard to tell! (but yes, partially because it mixes with many variables).
There is also a part of science of "we all WANT what makes us happy to be deemed healthy!". Decision affirmation rocks! So if there are 4 studies that say a drink a day is bad for you, and 2 that say a drink a day is good - which makes the newspaper? Coffee, chocolate, alcohol - all things people love, and all affirmed by at least SOME studies out there. Awesome!
