I found your question interesting, so I did a little research. There does seem to be a difference of opinion. However, for this diet it is actually only allowed in phase II of the Ultra Metabolism plan, which is after three week or longer depending on your goal.

Some of us are just streaching the boundries to keep on track with all of the other rules. Since agave nectar has a lower glycemic load, you can use it with out getting the rebound craving. Personally I try to use a dark honey for added sweetness because of it's antioxidants. I use agave when I need a milder flavor. Both of these are not without their faults, but a small amount, occationally is what I need to not feel deprived.

Maybe someone has a more technical answer, but I hope this helps.