The Ironman Way is that triathlon is an individual sport, and drafting gives you an advantage over the person in front of you that's unfair. Not exactly riding the same race if you're using 50% (I picked a number, that's not science) less energy.

I was reading somewhere (Triathlete?) it's apparently something like 7-9 bike lengths before the affects of drafting aren't obvious... but the point where you're crossing a line seems to be 3-4 bike lengths, and they'll only "get you" if it's pretty clear that you're drafting for drafting's sake and not just going up a big freaking hill as fast as you can, or getting ready to pass, or...

The whole "individual sport" thing seems to be the crux of those kinds of rules, along with people not being able to help you along the way. Race your race, as fair as possible.