Why will NASA pay SpaceX $843 million to plunge ISS into Pacific Ocean? Will it not burn up while making re-entry to Earth?

National Aeronautics and Space Agency has asked Elon Musk-owned company SpaceX to develop the ‘US Deorbit Vehicle’ that will lead the US sections of the ISS to its death when its time is over. But why? It could leave it to burn up while making re-entry to the Earth.