I don't know and had the same question about that.
JBoss Portal does not require anything, it's all about JSF that I am learning at the moment. If I find out, I'll tell you.
It may be a difference between the portlet controller and the Servlet controller. MyFaces uses the javax.faces.webapp.FacesServlet Servlet for a Webapp and the org.apache.myfaces.portlet.MyFacesGenericPortlet for portlets. Just a thought?