Not 100% sure but I believe these shrimp have a pretty complex life cycle and the egg's/larvae need to progress through a range of brackish water back to FW depending on what stage in development they are in.
Like I said I'm not 100% sure this is the case for these srhimp but it is for the majority of commercial available FW shrimp (chrry's, Crystal red shrimp and bumblebee shrimp are the only exceptions I can think of).