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The reason why is that concrete does absorb water and while it is very good at resisting compressive loads(i.e. your car), it doesn't like being pulled or pushed apart. What happens with rock salt is that you melt the snow into a slushy mess. This slushy mess allows water to penetrate the concrete. That itself wouldn't be bad, however salt has a property of attracting water so the extra salt content in the slushy mess causes your concrete to absorb more water than it normally would. Again still not that bad yet. The problem is that usually even with the salt, the temperature will drop down enough at night so that the water refreezes.
This is the problem, as the ice crystals grow and form they can tear the concrete apart from the inside out.
It's the refreeze after the salt that does the damage.
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