The default in the distribution is not to append - APPEND is set to "false". You can change this by modifying your conf/log4j.xml appropriately.
Maybe I don't see the big picture but it this a sane choice to do by default? Previous logging messages are lost when restarting the JBoss server...
A lot of the configurations are the default "development" settings. Both the JBoss and Tomcat/Jetty settings are defined in this manner. For a production environment, you do an amount of tweaking, including tightening of security. Sanne is working on a production deployment guide and I'm just finishing up the draft section on Apache SSL with JK2.
The setting is probably sane enough for a development environment where you don't necessarily want a large log accumulation during testing, particularly when you are stopping and starting the server a lot. I guess that is the rationale of the JBoss developers.