It's common sense.
Because playing shafts aren't made to withstand such powerful shots? Would you use winter tires in summer too? Playing shafts are lighter hence using weaker materials for less deflection and optical performance. Breaking shafts are made with stronger materials to withstand the large amount of power and to increase the power of the break. By breaking with a playing cue, you increase the chances of the cue warping and breaking.
If you've been doing that for 20+ years maybe your breaks aren't strong? (lol just kidding, but in all seriousness I would never break with my playing cue knowing my shaft costs $280) Besides what if your only shaft broke? You'd have nothing else to rely on.
His shaft was an old growth shaft which is a heavier denser shaft. As far as break shaft vs play shaft....sorry but most of the time it is just the taper. I use a lower grade maple for my break sticks that I make, as does 90% of the other cue makers.