lots of fish benifit froma little bit of salt, do you think every fresh water habitat has absolutly no salt in it at all?
That's simplifying it beyond any real meaning.
Though, to answer your question, there are many fish that develop in virtually salt-free conditions. Much of the Amazonian basin has salt levels that are only measured in parts per trillion.
The evolutionary history of the fish is among the bet clue as to whether a fish can tolerate salt or not. Freshwater fish are classified as either being a primary freshwater fish or a secondary freshwater fish. The primary freshwater fish have spend their entire evolutionary history in freshwater fish and hence never developed metabolic mechanism to deal with salt. Tetras and corydoras are good examples of primary fish. Secondary freshwater fish have spent some time in their evolutionary history in salt water conditions, and hence while the metabolic mechanism may not be perfect, they have some mechanisms to deal with salt. It is vestigial (like mankind's tail bone). Cichlids are a good example of secondary freshwater fish.
Now, certainly these are broad categories and there are exceptions to the general rules. Lemon tetras come from a branch of the Amazon that has moderate mineral contents and aren't as intolerant of salt as cardinals are, for example. Discus do come from a branch of the Amazon that is very mineral poor, and despite being a secondary saltwater fish a cichlid, they aren't very tolerant of salt at all compared to many other cichlids.
But, the big thing is that one amount of salt in every tank isn't right. Some species will be fine with it, others are going to be very intolerant. And, if all their tanks are linked together to a central filtration system (like a lot of stores are) it probably isn't a good idea to put a lot (any?) salt in at all. The freshwater species that can deal with salt are almost always good with low levels of salt, but the intolerant species almost never are good with salt.
And, to SJ2K, it isn't the scale/no-scale issue. Corydoras do have scales, they just aren't the same as the ones on a tiger barb or a tetra or a cichlid. Corydoras are intolerant of salt because of their evolutionary history, they have never spent any time in an environment with significant amounts of salt. The Amazonian basin is almost wholly very acidic and mineral-free.