By "fad diet", people usually the "cool new thing", and it pretty much is. Paleo diet and lifestyle is more or less a recent trend in the mainstream. But the people who equate "fad diet" with "bad diet" usually don't have an open enough mind to actually look at the science behind them. Most "fad diets" are pretty dumb (grapefruit and coffee diet, anyone?), but paleo is pretty well rooted in science. And not because it "works", but because many proponents will agree that paleo is more or less the most effective way of eating in the long run. If you eat just meat and plants and spend more time outside, no need for discipline. Just react to your cravings (which are dependent on the season and your activity level), and you should be fine.
My point? Fad diet or not, it's what is best for us. It is not a means to an end, but a way of life.