Well yes, you don't want to overdo with the salt, but in fact it's the water that acts as the heating element in that kind of a boiler (some steam irons work the same way), so unless there are ions to carry the current, the water doesn't heat very well.
Now that you mention it, I do see what looks like coiled cording under the foil, so it makes sense now. Thanks for clearing that one up. ;c)