all the scripts are easy to write, read. Even if you know nothing, you can still copy&paste - voila it's auto pk'ing some1 now!
but all the scripts must be interpreted, and it is the script interpreter that actually executes the scripts and fulfills the purposes of the script. No way can a script interpreter be easy to build.
It's practically impossible for Blizz to prevent hackers from writing script interpreters, or prevent lesser hackers from writing scripts on top of those script interpreters, or prevent lamemans from copying & pasting them. But technically, all scripts must be interpreted, and the interpreters are technically always detectable.
Mousepad's claim is true only to the extent that he has successfully cracked a particular released version (or versions) of the d2 client program. But observe this: every new patch (and even minor fixes) will automatically break Mousepad's code, forcing him to find a new way of hacking into the same d2 client. (Also note the code break is automatical, it may be something that Blizz desires, but it's just a side effect, not really a result of Blizz's coordinated efforts.)
Technially, Blizz could raise the bar so high making clinet-side hacking too costly to be worth the time.
But we all know, D2 = 2D = a dying game = Blizz won't intentionally break any "harmless" hack (Mousepad's materpiece is one of them).