Seems likely this has to do with money laundering. Even if bitcoins themselves aren't regulated, the exchange of them for illicit goods is. While the Silk Road or whatever to buy drugs is something I can see being frowned upon, profits from sex slavery and other human trafficking or mob activities would be the real problem. Also taxation. I suspect the exchange of bitcoins for goods in the US isn't going to be as easy as some people thought.
Actually DHS takes the lead on money laundering investigations. That would be ICE, part of DHS. And remember that the Secret Service is also part of DHS.
But DHS/ICE/HSI's mandate is so broad it's difficult to know what they're targeting here. Could be money laundering, could be something else. Their mandate also includes drug smuggling and computer crime, for instance.
Interesting, I'm just guessing. Since Dwolla operates in the US, from the recent regulatory announcement targeted at bitcoin(or any online currency), I'm just thinking that Mtg.OX not registering itself in the US, it won't be able to do business directly with US entities. The Ars article http://arstechnica.com/tech-policy/2013/03/us-regulator-bitc... would lead me to believe the money laundering is the most likely aspect to run afoul of regulation at the moment.
However, I have little insight, it just seems the most likely reason to me.
While I understand they are just nuking the biggest fish in the pond, I don't know of many people using their gox accounts as wallets. Most people I know had personal wallets they would move gox money into and out of if they wanted to spend it.
