Does salt prevent water from absorbing nutrients and falvours from food?
As a chemist, I'd say that you have it all wrong. 

You add salt to pasta water to have the salt infuse into the pasta. So as the dry pasta absorbs water, salt comes into the pasta too. Salted pasta tastes better than unsalted pasta.

Salted soup tastes better than unsalted soup. Salt enhances our perception of the flavors in the soup, but it does not extract the flavors.