The jbpm.sar is a service archive for JBoss, so that ou can use jbpm on top of J2EE/JBoss components without to configure hibernate/databases for each application. The sar archive contains a datasource that u can reference inside you applications. It also contains the libraries that you need (jbpm+jbpm-identity, hibernate.cfg, etc).
About the curious thing that is maybe because your application has the configuration files in its scope (hibernata,etc). You don´t need JBoss for using jBPM. If you have all the configuration files needed within your application´s scope everything should work fine.
Hope that helps