Well if the tag says so, and they didnt look healthy, chances are the species they had were brackish so yes you would need salt over the long term. 'Ghost shrimp' can refer to many species, some freshwater some not.
If this is the case, it is important to realise that they salt they require would be marine salt mix (like for reef aquariums), not regular 'aquarium salt' which is just over prided sodium chloride (like looking salt) and wouldn't provide any of the minerals/water stats that the shrimp need. I noted this because a lot of places tend to put things like "benefits from the addition of aquarium salt" on the tags, which would just be a waste of money, and in some cases can potentially harm fish over the long run.