When using the form tag for portlets, prepending the portlet's context is not wanted since we want the url to resolve to the portal's context and not the portlet's(where the form tag lives). The portlet url tags generate portlet urls within the jsp like /uPortal/f/s70/p/portlet-name/normal/action.uPpP_javax.portlet.action=saveItem
which resolve to /uPortal and not /PortletName
The original idea was to have the adding-context functionality off by default and turned on by an added parameter but it wasn't implemented. That way it wouldn't break anything while letting new apps take advantage of the shortcut.
I've managed to find a workaround for this by having the portal call WebUtils.exposeForwardRequestAttributes() before forwarding to the portlet application. This causes the form tag to use the portals context path instead of the portlet applications path.