“why the ICO is used to shutdown engine as opposed to the mag switch.”
This is used mostly in older designs of engine from the USA. Russian engines have always been stopped using the ignition system, as are Rotax, VW (most versions) and most modern engines using FADEC etc. Remember back in the down of motoring car engines were fitted with mixture controls and used magnetos, which were quite popular in the 1920’s.
Rod1