The reason they tell you not to use a hub is you have to aim overall advice at those with the lowest amount of common sense. Tell people they can use a hub, and some idiot somewhere will overload a passive hub with half a dozen high powered devices and complain at the manufacturer for saying it's ok to use a hub. That's the reality we live in. It's the same reason why in my office at work we are technically not allowed to plug anything into the power-strips built into our desks except the office PCs / monitors. The strips have 5A fuses, and every winter people kept blowing their desk by plugging 2kW electric heaters into the desks. So the official line is 'don't plug anything in except the office PCs' (but I have my phone charger plugged into mine with - of course - no issues).
Every time you make something idiot-proof, they make a better idiot. So it's best to issue all-enveloping 'do not' advice in the hope the dumb ones will just follow it blindly and the smarter ones will read into it.
