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.

Liberty Reserve, however, was shut down for reasons on the flip side of that coin: there was evidence that it was actually involved in laundering money.

Is this bad news for BTC? No. 

The real lessons here are: don't launder money (thanks, Liberty!) and register your unlicensed transmitter business (thanks Mutum!).