A complete wild-ass guess -- those concerns are most important in small-scale reactors, and the primary driving force behind small reactors is Navy submarine use. At sea you are surrounded by water, so there is no reason not to use it as your fluid. Adding salt would add another complication to the supply chain.

I guess you could theoretically pull salt out of the water, but there is no telling what impurities may be left. Reverse-osmotic water filtering makes water so pure that it supposedly extracts minerals back out of your bones and teeth, so impurities are no concern when it used as an exchange fluid.