Table salt is just sea salt (the stuff mined from underground just comes from an ocean that dried up a few million years ago) and is full of other compounds beyond NaCl.
EDIT: I looked it up. It's supposed to stabilize the iodide. The reason they use corn sugar is that it's cheap. Thankfully there are brands that don't include additives in their salt. I'll have to keep an eye out for them. I already get plenty of iodide in my diet.