Particularly when armed with a court order, the government has broad authority to seize property that can either be traced to the proceeds of a crime, or that can be shown to be used in furtherance of a crime. This comes up all - the - time in drug enforcement, where the police use it to seize vehicles used to run drugs. All that appears to be happening here is that the government is applying it to copyright enforcement.
The Libertarian magazine Reason has been covering the increased use (and misuse) of civil asset forfeiture for years now. It ticked up during the Bush years, and has been validated multiple times during the Obama presidency.
In particular, note that there are due process protections with strict statutory timelines attached to them for challenging civil seizures. One doubts many of these enterprises will avail themselves of those procedures; most would probably lose, and it's simpler just to host offshore.
"Why, oh why, is DHS and ICE doing the RIAA's work for them?" Because it's part of their charter. The "C" in ICE stands for Customs. Customs deals with counterfeiting (it's an importation issue), and hosts a large IPR enforcement division. No, the RIAA has not wheedled its way into DHS and refocused homeland security on copyright enforcement.
Civil asset forfeiture can get very sketchy. I wish there were more bright-line limitations on what the government can and can't do with it. The impression that I have is that in the majority of cases where it's applied, you generally want LEO's to have the power to seize what they're seizing. But every once in awhile, they pull some 60 year old guy over with the cash down payment he was going to make for his new pickup truck, declare him a drug dealer, and end up pocketing the cash because he can't find a lawyer. That's a miscarriage of justice. In light of it, I have a hard time feeling too sorry for people who lose their pirate music storefront sites.
I like 'daeken (I work with him!), and I know and respect that he has a very different take on intellectual property than I do, but his take on this seems a bit breathless.