Just like when making sauerkraut, soy bean mash is submerged in a brine solution while it ferments. The salt acts as a preservative.
And a more basic answer to the question is that in Asian cooking, soy sauce is used instead of salt.
So it's intended as a seasoning.
Some types have less salt than others. For example, Chinese premium dark soy sauce has a richer flavor and is lower in sodium than other types.
One thing I would suggest is avoiding brands like "La Choy" (or any other brand where "Hydrolyzed Soy Protein" is in the list of ingredients) which doesn't rely on fermentation to make the sauce, but rather takes the quick route by adding concentrated hydrochloric acid to break down the soy beans, and then adds an alkaline ingredient to neutralize the acid. After this, artificial colors and flavors are added. Yuck.