I found out why I had these empty-table-problems. I told JBoss about my table- and field-mapping.
JBoss seems to ignore my jaws.xml files. I added the default <create-table>false</create-table>, but JBoss keeps on creating them again and again :(
I put the files in /META-INF/jaws.xml in every JAR-file which contains an entity bean and bundles everything in one EAR-file. As far as I understand the whole thing, this should be the place to put them.