Think of it this way, waterbug. Why do you fear becoming dehydrated more than you fear hyponatremia? The answer probably is, because you have heard of one but not the other.
This article http://www.naturalnews.com/022477.html
says that hyponatremia is uncommon, but I would imagine that in fasting we don't have anything coming into our system, so our poor bodies have to work hard to keep the balance.
To be perfectly honest I had not heard of hyponatremia until I googled "opposite of dehydration", I just guessed that our age old bodies get the balance right using the thirst mechanism, and there must be a "too much" as well as a "too little" scenario, and we humans in the 21st century manage to screw up the balance by false beliefs.
Me, I always go back to basics and trust my body. It's a reliable little machine. I trust my instincts to a book any day.