There is a difference between "junk" foods and fresh veggies in the way they affect the body.

The "sweetness" of those processed low carb bars can trigger insulin release even though it isn't sugar. Which is why Atkin's tries to get people to completely eliminate those sweet treats out of your menu for 2 weeks. It is an attempt to restore your bodies reaction to releasing insulin.

As for high fiber cereals. How much sugar has been mixed in with it? Your mileage may vary. Only thing you can do is test and find out what your body's reaction is to certain food items. If you find you can eat things without weight gain then things are wonderful. If you find you stall out, then you know that food is to be banned.