Why?

Why the radio power wiring in any car has ACC power wire instead of having IGN power wire?

In other words, Why is it designed for the radio to turn off while cranking the car, and not on all the time when the key is not in the off position?

You want as much power available to start the vehicle, same reason (on most) vehicles - no accessories will function when key is in the (momentary) "start" position. 

In many vehicles there are two "IGN" sets of wires, in this case one set is to the battery, and the other is off an output from a relay connected to the alternator, so that when it is producing voltage,  certain systems are enabled (ie: ABS system). 

As for it operating all the time in the "run" position - if it doesent, there's more then likely a wiring issue somewhere.

Its late and Im tired, I hope I understood your question properly (?)

It sounds like right to have all the power just to crank the car, but really how much power a head unit would take, 10A?  so is that too much to take away while starting the car?

The reason I was asking, is because when you hook up a DVD player in the car, and tapp into the ACC power wire of the head unit, then if ur kids are watching a movie while the engine is off, and then you decide to start the car & keep going, then the DVD resets, and you have to start the movie all over.  not fun ... right?  So, usually a DVD player should be hooked up to an IGN source in order to avoid such a problem?

what do you all installers hook it up to? IGN in the ignition switch wires? or ACC behind teh ehad unit? 

Thanks for calrification.

Is the memory (12v + constant) connected correctly. It has been my experience that you should be able to pick up on the movie right where you left off, when interuppting power.

Another reason why electronic accessories are switched off during the starting sequence is because of the voltage surge that takes place from the starter's current draw. I have always advised the customer not to start the car with the audio system turned on. I have seen a few tweeters loose it from the voltage spike.......