Salt does actually hinder a lot of parasites' growth. If you have ever looked at microrganisms under the microscope... and then added a drop of dilute salt solution near the edge of the slide... well, you know what salt does to their cell membranes! (I am slightly embarassed to admit that we used to enjoy doing this in 3rd grade G&T class when we used the microscopes to look at pond water)
It is the same reason that we add salt (or vinegar or sugar) to the foods that we store (dried or canned). So yes, it is good for fighting parasites because it just makes it a lot more difficult for them to grow (this is what salt baths are for--they literally blow apart the membranes of these cells and kill the microrganisms).
When I used to work in a research lab with my ecology professor as an undergrad, we used salt baths a couple of times when our gupps got a tough case of flukes--if it doesn't kill the fish, it sure kills everything else!