It’s not about the global or countrywide scale. It’s about the local scale. If you take a cup of salt and eat it, it’s going to end back up in the ocean eventually, but it’ll make you sick before it gets there. Dumping salt into an area is going to screw with the ecosystem in that area, in a major way. We actually have similar problems in many areas due to stuff like fertilizer runoff from people’s lawns during rainstorms, causing toxic algae blooms in ponds and around beaches.