>4) If you put a frozen pizza in a cold oven it will always be burnt because by the time the inside is cooked the outside has become overcooked and burnt

This seems backwards. The hotter the oven, the less time for the heat to diffuse inwards before the outside is burnt. The lower the temperature, the more even the heat.

I'd imagine the reason not to put it before it's done preheating is standardization. If my oven takes 5 minutes and yours takes 10 to get to 400, there's no way to write one single set of directions for how long to cook it from the time of powering it on. It's hard enough with everyone's different freezer temperatures.