Atkins described his diet in his first book as "high calorie". Taubes describes low carb diets as unrestricted calories. While unrestricted is not an incorrect interpretation of ad libitum, it is misleading. Most people will equate unrestricted with higher intake when that plan is compared to a calorie restricted plan. The implication is that those on an unrestricted plan are eating more than those on a restricted plan and therefore are losing the same amount of weight (or more weight) eating more calories on LC vs. other plans.
If I had a dollar for every time I've read something along the lines of "just imagine how much more weight the LC group would have lost if they were allowed to eat more", I'd be rich! Clearly too many interpret ad libitum incorrectly.
This study I've previously blogged on is but one of many examples in the literature of spontaneous reductions in intake when allowed to eat as much as one wanted. In this study carbohydrate was maintained at 50%, and weight was maintained for 2 weeks on 35% fat and 15% protein, then maintained at the same caloric level for two more weeks swapping out fat for protein - 20% fat and 30% protein. The participants were then given an excess of food but told to just eat what they wanted of the 50/20/30 diet. Guess what happened? They lost weight. Why? Because they reduced caloric consumption by ~440cal/day on average.
Ad libitum LC diets promote caloric restriction. This is more "natural" IMO and one of the better aspects of low carbing. This wasn't a LC diet, but when told what to eat and when, you are necessarily overriding your personal signals. But ad libitum means listening to those signals. This, my friends, is a *good thing* (said in my Martha voice) for weight loss and the long run.