Sodium chloride is natural, but 'tonic' is a very loose word so aquarium salt companies do tend to throw it round a lot .
It's no different from adding tiny amounts of cyanide and calling it a 'natural tonic', because cyanide is just as natural as Sodium chloride, and in small enough doses it will do no harm.
Though technically there is a chance adding aquarium salt even in the tiny recommended doses could do some harm over the long term, nobody has really studied it because nobody in a position to do so has ever needed to - big fish farms and big commercial fish breeders know it's useless, so they don't use it and therefore have no reason to study possible negative effects.
And also, even the small doses of Sodium chloride the boxes tell us to add cant be considered natural, because the environments most of our aquarium fish (especially bettas) naturally come from contain much less Sodium chloride, in fact most peoples tap water contains more Sodium chloride than in these fishes environments.
There is the exception of fish that naturally come from slightly/fully saline water, or extremely hard water - but for these fish you don't want to be adding just Sodium chloride, you want to be adding marine aquarium salt - which is just around 70% odd Sodium chloride.