Zend certified PHP/Magento developer

Why does Magento list posting an item to cart under “Non-cacheable page checklist”?

I was reading this and I think I am misunderstanding something. It list this, “Use POST requests to modify Magento state (e.g., adding to shopping cart, wishlist, etc.)” under Non-cacheable page checklist.

Why would this invalidate the page being cachable? The page is still cacheable, the DOM is just being manipulated with js?