Postgres can't be run as root. From src/backend/main.c:

    if (geteuid() == 0)
    {
        write_stderr("\"root\" execution of the PostgreSQL server is not permitted.\n"
                     "The server must be started under an unprivileged user ID to prevent\n"
          "possible system security compromise.  See the documentation for\n"
                  "more information on how to properly start the server.\n");
        exit(1);
    }