The problems are:
1 - Scientists know what studies are for, which is primarily to point the way to the next study. It is impossible in almost all cases to account for every possible factor influencing outcomes, so the results of studies can only ever me seen within the strict context of that study and how it was designed. Scientists often so take that for granted that they fail to fully recognize that their results will be misinterpreted and that people will assume too much. They will, indeed, sometimes even make statements that sound like they are declaring the definitive answer, but it is ALWAYS implied that it's all tentative. Confronted with being taken as definitive, they may well respond, "But it was a study, and everyone knows that's all it is and that when I say it shows such and such, I mean that such and such is true ONLY if the results turn out to be truths in the future."
2 - Journalists are often rather poorly educated and are not often trained in any scientific discipline. If mostly you took journalism classes, you weren't learning much about anything else. So they don't know how science works and that studies are just steps toward the next study and that it is fully expected that conclusions will change. If it weren't true that conclusions would invariably change, science could close up shop and go fishing, since there would be nothing new to learn. But it makes a better story that so and so causes obesity and an even better story that so and so turns out to not cause obesity. Neither is the final word and was never intended by the study's author to be so. The journalist seeks story, not truth.
3 - Critical thinking skills are even more neglected than science education. When someone believes in things that they don't understand, they're at risk of accepting a magician's stage trick as reality. The journalist waved his hands and turned tentative knowledge into definitive fact, and without reading the original paper and without possessing a understanding of how science works, the reader can't possibly understand how he did it and therefore shouldn't take it as reliable fact. And in the case of more intentional deception, follow the rule: Never automatically believe anything when the person who tells you has anything to gain from your belief.
The Internet has caused people to be more ignorant than ever in human history, in terms of the shear amount of patently false things they believe to be true. Primitive man may have believed wrongly about nearly everything, but the everything didn't amount to too much. Pre-Internet, people had to make considerable effort to acquire bogus knowledge beyond the evil effects of their local newspapers and schools. Now, all the false, self-serving, and plain loony "facts" of the world are at one's fingertips.