The title is a direct quote from the Dwolla terms of service.
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Dwolla may only be used in connection with United States Financial Institutions: User funds must originate at a United States Financial Institution, and Dwolla will only instruct Veridian to transfer funds to a United States Financial Institution associated with the appropriate Dwolla User.
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This paragraph prevents using Dwolla for MtGox (not a US institution) Withdrawals. Why then does depositing money from MtGox into Dwolla still work?