Because petco keeps disgusting disease-ridden dime-store stock fish from asia and salt is a cheap band-aid to keep most parasitic and bacterial activity at bay. If your fish are healthy they do not need salt. That said, Byron saved your butt on keeping your beneficial bacterial alive but I must disagree with his stance on livebearers and salt. I have bred guppies in soft and hard, and even in brackish water and they grew faster and larger with better coloration not in the soft or the hard but in the brackish water. I'm not suggesting you kill your beneficial bacteria by switching to brackish, but just want to open up the idea that guppies do well in marine salt. I have not used the pure non-marine salt alone with guppies so don't have data on that at all.
Best of luck with your fish!