I don't understand this either...is this just for running or all exercise? I mean, I tend to do the elliptical and stationary bike the most and stay between 85-90% of my max HR for around 20 minutes (during a 30 minutes workout, excluding warmup and cooldown). And I'm doing this on the Fat Fast, which I'm guessing definitely doesn't have enough carbs.

See, I'm the exact opposite - I lose the energy to sustain a max heartrate if I do eat carbs. If i don't, I'm fine. I think its all what your body gets used to. I'm definitely a person who cannot eat carbs - they make me very lethargic - even before I ever low-carbed. When I started on induction the very first time, and any time I "fell off the wagon" I never had the detox flu or anything - I felt GREAT as soon as I stopped eating carbs. I remember the first time I did induction - I never realized how crappy I felt all the time before I cut out carbs!
I think there's a certain degree of mental intimidation involved as well - people tell you that you NEED carbs to be healthy, etc - so its ingrained in your head that you won't feel good if you don't eat them.
I would say that since every person is different, you may do just fine without the carbs, like I do, while others won't.