Accessing View State (the JSF Component Tree)

That component is part of the view state, which is only available to Post requests. ( ... )

Therefore, the zipChanged function generates a Post, rather than Get, request when the fifth zip code character is entered.

Also, note the URL that is sent in the request:

    function zipChanged(zip) {
        if(zip.length != 5) {
            hideErrorsDiv();
            return;
        }
        new Ajax.Request(window.document.forms[0].action,
           {       method: "post",
               parameters: "ajax=true&zip=" + zip,
               onComplete: processRealtimeValidation
           });
    }