It isn't the protein, or anything else in the bars that make you gain weight-calories are the key factor. To lose weight, you simply take in more calories than you burn each day. (By eating less of them, and burning more of them by exercising and being more active.)
Protein bars and powders are on the market because active people, those who exercise regularly, need more protein than those who are sedentary. Protein is the fuel for your muscles, (just as calcium is for the bones and teeth).
If you don't take in enough protein, then your muscles don't have the fuel that they need to maintain and/or grow. If you eat a diet that doesn't contain enough protein, and you are dieting and exercising-then you are not giving it the fuel that it needs.
The idea of exercising is to gain muscle, and lose fat. This is how you go from jiggly/flabby, to firm and shapely. The exercise works the muscles, and burns fat-but in order for it to work you also have to fuel the body properly.
You don't have to use protein bars or powders, as long as you eat a diet that contains enough protein-eggs, chicken, fish, lean beef or pork, soy, dairy, etc.
Protein bars and powders are a more convenient way to add extra protein in the diet, if you need it.
The bars won't make you gain weight, if you are staying with a low calorie diet suitable for your weight and activity level.
Remember-you want to increase muscle, and burn fat, while you lose weight.
Start weight after baby #3: 233.9
Current weight: 143
90 pounds lost!