They don't consider it mandatory because the TJ brakes are all over the map. I have literally driven a TJ (more than 1 or 2) on 37's that would lock up the tires with stock brakes. I've also driven them on 33's that were dangerous. The reason we see the varied responses as to what is and isn't needed is due to folks reporting how what they have works and that is so widely varied as to not be a given.A lot of people also wouldn't consider a brake upgrade mandatory.
However, having driven with 35s and stock brakes, I can tell you that it's a very scary ordeal if you ever have to really hit the brakes.
If someone tells me their rig will lock up 35's with the stock stuff, I never doubt them.