Criminal intent
According to TFA, it seems these people were potentially operating an illegal money transmitting facility.
In USA the following applies:
http://www.law.cornell.edu/uscode/text/31/5330
http://www.law.cornell.edu/uscode/text/31/5313
One of the requirements is that "When a domestic financial institution is involved in a transaction for the payment, receipt, or transfer of United States coins or currency (or other monetary instruments the Secretary of the Treasury prescribes), in an amount, denomination, or amount and denomination, or under circumstances the Secretary prescribes by regulation, the institution and any other participant in the transaction the Secretary may prescribe shall file a report on the transaction at the time and in the way the Secretary prescribes. A participant acting for another person shall make the report as the agent or bailee of the person and identify the person for whom the transaction is being made. "
These laws were put in place to control money laundering.
That pretty much forbids any anonymous transactions, whether for drugs or legal products. While they cannot nail the anonymous people, they can nail the people providing the service.
It might not be morally right to cuff them, but the law is the law and at this stage it looks like they might have broken it.
If this is run through the courts as a test case, it could potentially break Bitcoin in USA.