Salt and sugars are natural parts of our environment and salt is actually a necessary part of our diet. We have organs evolved explicitly for the purpose of maintaining homeostasis of salt and sugar, namely our kidneys and pancreas.
What do you think happens when you overheat animal fat or oils in a cooking vessel and "season" it? It forms a plastic. We've been living and eating alongside that type of plastic since we learned to cook food as a species.
For plastics we have no evolved tolerance.