The death knoll for slugs and snails is summed up in one word:
Slugs and snails have a higher percentage of their body-weight made up of water, then other animals. Also, their skin is a much more permeable covering then average.
When you salt a slug or snail, this increases the concentration of salt on the surface (outside) of the unfortunate subject. From a different perspective, this is DECREASING the water concentration outside the slug/snail.
The process of osmosis is the movement of water from an area of high water concentration through a permeable membrane to an area of lower water concentration. The water INSIDE the slug MOVES OUTSIDE the slug, in order to reach an equilibrium concentration on both sides of the skin.
Unfortunately, this means that the slug/snail is now much drier then it can tolerate. It literally dies of dehydration.