That is a common misconception on mollies as they can strive in BRACKISH or FRESHWATER, I have mollies in my 100% no-salt tank.
That's the common misconception, actually. Mollies don't do so well in freshwater. Sure, they can live in it, just like you could probably live in a closet your whole life, but it wouldn't be the best for you, or make you the healthiest. If you are going to keep Mollies in freshwater, it needs to be hard and alkaline. That's the kind they prefer. If it's soft and acidic, that's when you really need to add salt. Wild type Sailfins and Black Mollies are the ones that just need it, however, almost all Mollies are hybridised so it's virtually impossible to look at them and say "You need salt!". It's best to keep all of them in a bit of salt, to make them the healthiest and happiest they can be. In pure freshwater, they are much more susceptible to disease, especially shimmies, and they don't tend to live as long. Salt is the way to go with these guys.