Yeah this really surprised me too. I would have expected the majority of SPAM to be sent from Russia et al. I suppose that's why it has taken so long for anyone to get around to shutting down this California company.
As far as whether or not they've broken any laws... They have undoubtedly been contacted countless times with spam complaints about their hosted servers but have ignored them. If they were not "bulletproof" to some degree they would not have attracted all the spammer business to begin with.
So perhaps there isn't a specific law that is being broken but it seems likely a lawsuit would succeed.
Industry experts love to tell people things :-)
No offense meant but, if you stop and think about it, that it makes no sense. For a 75% drop in spam overnight it had to be the servers that were actually sending the spam that went down.
It also follows that of the remaining 25%, only a small percentage comes from zombies.
Which makes sense as you can send a lot more email, more reliably from a few disposable dedicated servers then you can from a zombie network. Also these days it's cheaper to rent servers than it is to rent zombie networks.