Why can the pilot shut off the fuel during takeoff?
Fire, probably. But also, how complicated would you make the system if you needed to prevent certain switches from working during certain times of flight? At some point... we're all just in the hands of the people in the cockpit.
I can't put my car into reverse gear while driving down the freeway.
Remember the "surging" incidents where the driver insisted he was stepping on the brake but was actually stepping on the gas?