Any animal (snail) that has a hydrophilic (water loving) skin will be affected by direct application of salt.
They experience a process called desiccation where water is drawn out of their bodies, at certain point enough water is drawn out to result in death.
The opposite is also true stick these creatures in water with a higher purity then they are used to; and they will intake enough water to kill them as well.
Salt does not "burn" the snails. Its basically the process of dehydration. The water concentration on the outside of the cells is low and when the salt is poured, the water from inside of the cells comes out through osmosis. Thus snails experience dehydration (loss of water) and die. The water moves outside to neutralize the concentration of salt.
Hope this helps :)
Snails have a mucus membrane all around them that helps them move (the slimy trail you see behind the snail is this mucus). When salt is added to the liquid mucus it draws more water out of the snail itself drying it out. It's the same reason salty food makes you thirsty.