The transponder in the key still has to communicate with the ECU and handshake the security protocol. The start button is merely there to send a digital request (over CANbus network) to the computer to control the starter/ignition/accessory wires I assume. Ever notice when you get near the car with the key there are some relay noises, whereas I assume it is the computer engaging the fuel pump.
Check this out..
http://hackaday.com/2012/07/07/keyle...easy-to-steal/