not ALL red clawed crabs require brackish. the most commonly sold red claws, sesarma bidens, does need brackish. From time to time, other species that look like sesarma bidens either are imported by accident or are contiminants. They most likely do not need brackish because the sesarma complex is mainly freshwater. They also do not require land as much as the fiddler crabs, but would like it very much to climb up onto something when there are territorial disputes.
there is a family of crustacions from south america in the genus Aegla which are not really crabs and not really crays/lobsters. Those are truely freshwater although they are not seen very often in the hobby (sometimes they are on aquabid, but they are really expensive... like $60 each)