Use of salt as preservative
High salt environments are strongly hypertonic and disrupt the osmotic gradients of cells.  Excess salt gets forced in (disrupting function) and cell water gets drawn out.  

In simpler terms, it dehydrates them.

This link has more info regarding what is required to live in high salt environments (and by corollary, why organisms without such features die):

http://en.wikipedia.org/wiki/Halophile