RTFA and you see that, as many of us already know, you can get a court order to get the exact identity of the account holder, so the problem as described by the summary quote is not the real issue. Rather, just because you know the account holder does not mean that you can prove that the account holder, or whoever you have on the stand, is the one that infringed.
Despite rear-end covering clauses in the terms of most home ISPs that state that the account holder is liable for everything that goes across their connection, most courts won't accept that. I wouldn't be willing to test it, but it's a very valid point of defense. The number of people with open Wi-Fi is staggering, and even then there are attacks which work on WEP (a ton) and WPA (GPU accelerated attacks can get passphrases in under a minute on many routers), which is the maximum security many home routers in use are capable of. That makes this point even more valid.