It seems that this is the default behavious jboss has adapted. I have faced the same problem trying to exploit the DB Constraints was not able to as JBoss before deleting updates to references to null .. and then it calls the remove.
There is nothing configuration wise available right now to make JBoss understand to not update the foreign key references.
Are you using the jboss relation tag with fk-constraint set to true ?
It is the intended behaviour.
Thanks for your replies. In the case where I have a restrictive database constriant(one that prohibits the deletion of the parent if it has any children), I will implement the fix in code.