The US authorities seized the funds in Mutum Sigilum's Dwolla account because Mutum Sigilum was operating an unregistered money transmitter business. The registration requirements seek to prevent money laundering, but there was no evdience (at least that I'm aware of) that Mutum Sigilum was actually laundering money.
Mutum Sigilum was allegedly operating an unregistered money transmitting business, but there isn't much evidence of that either. The informant in the case only sent money back to himself. And even if Mutum Sigillum was operating a money transmitting business, it isn't clear that they are subject to the reporting requirements of section 5313 (which triggers the registration requirement) because such reports would be filed by Veridian and Wells Fargo.
The legal issues in this case are a lot more complex than you imply. If Mutum Sigillum was operating an unlicensed money transmitting business, then arguably Dwolla is too, since Dwolla is having Veridian transmit money for Dwolla's customers. You might also want to read:
http://k.lenz.name/LB/?p=9369It is not relevant whether mutum was laundering money. It only matters if mutum was engaged in money transmission (without a license). We already know mutum is not licensed, and neither is mutum's affiliate tibanne co. There is a weakness in the government's case here, though. It is not clear to me that mutum was engaged in money transmission. Tibanne probably is under federal guidelines, because it is an exchanger of virtual currency. Not so sure if mutum meets this standard at the federal level, and would have to look closely at MD state law to see if simply processing payments to/from Dwolla counts under state laws. Just a guess, but is it possible that the feds went after mutum b/c it is the account-holder and they wanted to be able to seize the account immediately? Seems like tibanne would have been the better target, except that it would've required that the gov't prove a couple extra steps between tibanne and mutum to go after any of the US accounts or assets.