The reason for not giving freshwater fish a salt water dip is this: the salt water is dangerous on a cellular level for the fish due to the change of the osmotic pressure in the cells.
Here is the article on fishlaboratory.com explaining this in the part about anatomy and physiology of the fish.
In freshwater fish the level of salt is higher inside the cells than it is in the water surrounding the fish so the cells keep the salt inside them.
If you dip the freshwater fish in water containing high level of salt, the salt water will pull the water out of the cells of the fish, so the fish gets dehydrated and starts to drink water to compensate for the loss of water from the cells, but this adds more salt making the fish lose even more water until it dies from dehydration.
Saltwater fish have about the same concentration of salt in the cells as it is in the water surrounding the fish.
In saltwater fish, if you put it in freshwater, the cells will have higher concentration of salt and the cells will absorb water, but the saltwater fish can then simply stop drinking and thereby stop adding dangerous levels of water to the cells.
In saltwater the fish is drinking constantly to flush out the excess salt from its body.
Fish that live part of their life in saltwater and part of their life in freshwater have special adaptations in their gills, as this is the organ regulating the water and salt level in the fish; the same goes for fish living in brackish water.
Here is another article on blogionik.org about salt regulation in freshwater and seawater fishes.
Edit: I had the transport of water to and from the cells reversed but this is now corrected.