It's about addiction, not just about knowing something is healthy. Many, many scientists (people with bigger brains than I) have stated that fast food
may be as addictive as heroine. One can become dependant on the high levels of sugar and fat found in fast food like one might find at McDonalds.
Yes, everyone knows that an apple is more healthy than fries, but if someone wants the fries more than the apple....you get the idea. As someone who is still battling with food addiction (mine was psychological, whereas the addiction to fast food is both psychological and physical), I can tell you it's not as easy as walking to Safeway instead of Burger King.