They actually gave the number lower down. “Roughly 40% of domestic salt, produced largely from mining, is used not for food or chemicals, but for deicing.”
So, I don’t know what the issue here as the term main use is as you say accurate.
I won't argue this specific case but, in general, terms like "most," "about half," a "significant majority," and so forth can be more readable for a general audience than spouting off a bunch of precise numbers (which may not really be that accurate anyway).
I agree that just writing "40%" would be more informative, but that's not how journalistic writing works, for reasons I don't fully understand.