Brine shrimp breed in very salty water, but you can hatch them in merely brackish. However they will live a couple hours in freshwater. Long enough to be found and eaten. Its a common practice to feed salt live food to freshwater and vice-versa. Its not what they eat in nature and you should rinse off the salt if its going in a softwater or planted tank, but it cuts down on disease transmittance. Brine shrimp are unlikely to transmit parasites to freshwater fish.