I dont know many people that shifts into Park at stop signs, apparently you do, though (know people that shift into Park at stop signs). It is, however pretty common to shift into Park in drive-thru lines, when waiting on people to enter or exit the vehicle, to talk to another driver out the windows (cop style).
In any case, in Park, most people aren't expecting or attempting to move, either forward or backward, so engine power usually isn't needed, just as accessories can be ran without engine power.
But again, driver stops, engine stops, driver shifts to Park, engine starts, driver turns vehicle off, engine stops. That's a bit redundant and silly, especially considering energy to start via generator or starter has to come from somewhere.