Saltwater corrodes firefighting equipment and may harm ecosystems, especially those like the chaparral shrublands around Los Angeles that aren’t normally exposed to seawater. Gardeners know that small amounts of salt – added, say, as fertilizer – does not harm plants, but excessive salts can stress and kill plants.
I don’t know, but if I had to guess like everything else it comes down to money. It’s energy intensive to desalinate seawater to the degree it’s drinkable, and now we’re talking about adding even more energy to refine it even further to make it suitable for human consumption. That makes any recovered salt expensive compared to natural salt deposits. Much easier (read: cheaper) to just scrape salt deposits that have already evaporated.