ColdFusion generated 403 response does not trigger IIS6 403 error page

Question

I am using the following line of code in several places in a ColdFusion website application, which will cause the page to respond with a 403 error code, but, IIS6 does not show the 403 error as specified in "Custom Errors". This happens whether I use the default settings, the built-in IIS error pages, or my own custom URL or File setting:

Application.cfc:

<cffunction name="onRequestStart" returntype="Boolean">
    <cfargument name="targetPage" type="String" required="true" />

    <cfif notAuthorized>    
        <cfheader statusCode="403" statusText="Forbidden" />

        <cfreturn false />
    </cfif>

    <cfreturn true />
</cffunction>

This code properly triggers the 403 error in IIS7/7.5, but not in IIS6.

Why is this happening and how can I resolve the issue?