Why can't Dwolla send to Mt Gox?
Mutum Sigillum LLC was the legal entity that Mt. Gox used for transferring funds using account-to-account transfers and for adding to (and withdrawing from) their account using bank funds.

Mutum Sigillum LLC is not registered as a money transmitter in any states.

It may not even be registered as a Money Service Business (MSB) through FinCEN.

FinCEN provided guidance that those exchanging "virtual currencies" (which they misapply, but intend to refer also to Bitcoin) for dollars are money transmitters and required to register.

This action by the DHS may be related to that.

Or it could be related to the Bitcoinica lawsuit in which Mt. Gox is holding funds of Bitcoinica customers that the court-appointed liquidator in New Zealand has requested access to.

Or it could be related to any number of additional factors, including a single illicit transaction in which the funds traveled through Mt. Gox, who knows.