The problem arrises in training aircraft because most of the aircraft in question were designed in the 50's and 60's when the mass of the average American was about 120-140lb's, now the mass of the average American (and Brit) is heading towards the 180-200lbs mark, meaning that we cannot take an aircraft full of fuel and passengers like they did when these aircraft were first designed.
My CPL instructor said for commercial reasons you mat have to fly slightly overweight but as long as you are forward rather than backward of the datum. He did say however it was entirely up to the commander what decision ws made.