Support for Atkins diet, Protein Power, Neanderthin (Paleo Diet), CAD/CALP, Dr. Bernstein Diabetes Solution and any other healthy low-carb diet or plan, all are welcome in our lowcarb community. Forget starvation and fad diets -- join the healthy eating crowd! You may register by clicking here, it's free!
I think it has more to do with the type of flour used than the sugar. The amount of sugar in a bread recipe is very small. I used to make a batch of bread that made 4 loafs and I think it was 1 tsp of sugar for the whole recipe. The rising of the yeast is also dependant on the gluten in the flour and the kneading helps to develop that gluten. Whole wheat bread does not rise as highly as white bread because it does not develop as much gluten.
A lot of the low carb breads use a different kind of flour and these flours might not develop enough gluten to act with the yeast. I don't know for sure but that seems a better reason that such a teensy amount of sugar for not using yeast.
Yeast will not act without a little sugar. It doesn't have to be a lot, 1-2 tsp. will do it. Sugar is yeast food and it consumes it in the process of growth.