Exercise has demonstrable good effects on the body, especially in our present environment. And I'm convinced Dr. Atkins was determined for people to succeed on his plan (bless his heart) and pushing exercise is one way to ensure that.
I plan to exercise more myself, because I enjoyed it and liked it when I could fit it into my schedule. And I think people are more likely to exercise when they are slimmer and feel better.
The big thing about pushing exercise, from the mainstream, is that it is the only hope of people burning up all those carbs they eat, and showing a weight loss on low fat/high carb!