Because unburnt fuel ends up running down the exhaust pipe into the Cat which could be warm if the engine has run recently. This warmth may be enough to vaporise the fuel without igniting it, thus you have a heavy concentration of fuel vapor in a confined space. When the engine eventually fires it could ignite the fuel soaked Cat with explosive results. I had it happen to me once and it opened up the Cat like a it was made of foil - as a mechanic i deal with more non-starters though! It's rare in reality - a lot of criteria have to be met for it to happen but the maunfacturers play safe and just say 'dont do it'.

On the subject of bike jump leads i have a set i made from a twin core 2mm sq 25amp wire and have found it to be adequate for bike-to-bike jumps .

Yes... easy
 

... it is!

Didn't expect it to be (in memory my old tuned SRX600 was almost impossible to bumpstart)...

... but all it took was a short push (on level and paved ground) in second gear and a jump on the saddle.

Battery had gone dead (at least: not enough juice to make the starter engine go round), engine was cold, but surprisingly enough it started with first attempt.

HJ