Imagine how badly the Primal Blueprint would work if the parameters were "eat as much of all these foods as you want" without anything to rein it in. You'd have all these sugarholics chasing down their ribeyes with whole watermelons. I'm sure that would work well for effortless weightloss.
At 100-150 carbs most people are going to a) limit the binge worthy stuff, b) maximize foods that fill you up for hours, and c) not suffer low carb flu or at least not for very long. The result is going to be weight loss for just about any adult who adheres to it.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Highest squat: 167.5 x 2. Current Deadlift: 210 x 3