XWork actions in uploaded plugins do not work

Plug-ins containg XWork actions do not get registered correctly. The plug-in lists as working, and the XWork module is listed, but when you try to access it, it doesn't work. However, if you move the same plugin.jar file into the /WEB-INF/lib directory, everything magically starts working. Try it with the livesearch.jar, for example, and then go to [confluenceinstallpath]/plugins/livesearch/livesearch.action.