It's simple, really. The more you sweat, the more salt you need to replace the salt you sweated out.
In fact, athletes - especially endurance athletes are in danger of a condition called Hyponatremia.
This is interesting. Basically, if you reduce salt intake a little and exercise more, you may lower your blood pressure.
If you are as unlucky as I am, you don't sweat much. People think I'm not working hard enough. I am. I just don't sweat very much. One of the few times I could get away with more salt was at the Coachella Festival. When you are sitting in 100 degree heat in the sun, there isn't much you can do in the way of staying cool. In fact, I did get dehydrated the first time I went.
I'm a bit jealous of one of my college friends who seems to be able to eat all of the salt she wants. She had a shaker in her hand on Christmas Eve. But, then, she plays ultimate Frisbee, goes hiking, and goes running. I can't really do those things.
This article from Rice University goes into more detail about the issues. http://www.rice.edu/~jenky/sports/salt.html