Here are some further details on their stance:

Dwolla does not receive, hold, or transmit User funds; Dwolla only maintains and manages information associated with User ownership of the funds; (3) Veridian is the entity that provides money transmission services upon instructions issued through the Dwolla software platform; and (4) Funds in the Veridian Holding Account are held in a pooled account.

Veridian, a major investor and bank processor manages the funds. In other words, all funds withdrawn from a user’s bank to be put into their Dwolla Account is placed into the pooled Veridian Holding Account. This legal loophole allows Dwolla to operate outside the state of Iowa, since it is technically never a money transmitter, merely a service provider for a processor, and therefore not subject to the expensive registration and licensing process.

http://sites.tufts.edu/costofcash/2013/03/27/dwolla/