c# - WCF for AJAX - "ordinary" web services: Where's the "d"?

Question

I am trying to catalog for my own education the multiple ways services are used in an asp.net application.

When using a standard (i.e. non-ajax) WCF service, if you use an endpoint behavior with "webHttp", a json result is not wrapped with "d". If you use an endpoint behavior with "enableWebScript", you get the expected "d" wrapper.

Is this a feature of .net 4.5 now, or why is it that you don't get the "d" wrapper with webHttp?

My understanding now is that it is the underlying MS-AJAX infrastructure that implements the "d" wrapper, and if you use WCF without the AJAX infrastructure (e.g. if setting up a RESTful service), you avoid this.

I don't have any real complex implementation - I am just trying to capture the nuances of the different ways of implementing services - but just in case, here is the code I used to discover this behavior with "d".

Here is the exposed method:

[ServiceContract]
public interface IServicesDemo_WCF
{
    [OperationContract]
    [WebInvoke(Method = "GET", ResponseFormat = WebMessageFormat.Json)]
    string ReturnWcfMessage();
}

Here is the config:

<system.serviceModel>
<behaviors>
  <endpointBehaviors>
    <behavior name="WebService.AJAX_WCFAspNetAjaxBehavior">
      <enableWebScript />
    </behavior>
      <behavior name="WebService.WCFBehavior">
          <webHttp />
      </behavior>
  </endpointBehaviors>
  <serviceBehaviors>
    <behavior name="">
      <serviceMetadata httpGetEnabled="true" httpsGetEnabled="true" />
      <serviceDebug includeExceptionDetailInFaults="false" />
    </behavior>
      <behavior name="WCFServiceBehavior">
          <serviceMetadata httpGetEnabled="true" httpsGetEnabled="true" />
          <serviceDebug includeExceptionDetailInFaults="false" />
      </behavior>
  </serviceBehaviors>
</behaviors>
<serviceHostingEnvironment aspNetCompatibilityEnabled="true"
  multipleSiteBindingsEnabled="true" />
<services>
  <service name="WebService.ServicesDemo_AJAX_WCF">
    <endpoint address="" behaviorConfiguration="WebService.AJAX_WCFAspNetAjaxBehavior"
      binding="webHttpBinding" contract="WebService.ServicesDemo_AJAX_WCF" />
  </service>
    <service name="WebService.ServicesDemo_WCF" behaviorConfiguration="WCFServiceBehavior">
        <endpoint address="" behaviorConfiguration="WebService.WCFBehavior"
          binding="webHttpBinding" contract="WebService.IServicesDemo_WCF" />
    </service>
</services>

Here is the call from client:

$.ajax({
            type: "Get",
            data: data,
            contentType: "application/json; charset=utf-8",
            url: "ServicesDemo_WCF.svc/ReturnWcfMessage",
            success: function (data) {
                $('#wcf_content').html(data);
            },
            error: function (msg) {
                alert(msg);
            }
        });