From my comment:
Personally I'm not following your question. Are you talking about a specific episode where you noticed they had salt but no explanation for where it came from? Or are you assuming they have this one resource replenished for whatever production reason on every episode? As far as I can tell - assuming the show is legitimate - they never have anything except what is explained in the episode be it brought, provided or found. The answer then is as many suggest: they just don't have salt.
To further the comment, the show is interesting in providing a comparable survival experience in many harsh environments. Some get a land of fruit, water, fish and more. Some get a hot barren desert. One episode, a guy was kicked off for trying to secretly raid the crew's snack pile. I don't have a lot of confidence reality TV isn't guided and to some degree a straight up lie. But Naked and Afraid has never given me the impression it isn't largely legitimate from the perspective of survival and resources... the bickering... some of that may be prodded by the producers. I've worked with some people who have been on the production crew of some reality TV shows and they say they range from complete lies to a normal scenario where the producers give them subjects to argue about. Fabricated drama, as it were. Because drama gets ratings.
From what I've seen on the show, I don't think the producers are stepping in with condiments or seasonings. I think they want the raw reactions of eating things like snakes, insects, and other vile things without the benefit of masking the atrocious taste. It makes for better television that way and costs less. Win win for the producers.