Good question, because it addresses some facts about EOS not well known. By using the blockchain, every participant agrees to the constitution and thus arbitration, and as for now no other forum has been defined, ECAF is the default arbitration, just as described by the constitution. So everything is there. And by starting the unstaking process on the nascent EOS blockchain, those scammers, did formally agree to arbitration.
If you are interested in this topic i recommend watching Hive account@dan's recent explanations how this works:

In addition: If in fact the arbitration-action would have been illegitimate, one of the current possessors of primary keys of those accounts frozen could have stepped up and dispute the arbitration ruling, but until now only uninvolved 3rd parties have done so.