Good answers here, but I think the main point has been lost.

You add salt to ice to make it melt faster, which in turn will cool the dish that has the ice cream in it.

If you add water to the outside shell, it would cool down quickly, but not get that cold, because water is warmer than ice.

However if you put ice in the shell and add salt, it will begin melting faster, which will allow the cooling process inside the shell to occur more rapidly.