The target companies dutifully add a few words to the proxy and hand the lawyers enough money to pissoff, which they gladly do since actually litigating a case involves work and some legal talent.
There's all kinds of corporate-lawyer idiocy at play here: MobileRead was just linking out to another site hosting the script, which can't actually be used to break Kindle DRM, and the only people using it are the people who want to buy more books -- not exactly the sort of customers you'd want to pissoff.