I think you got it. It's just to verify that the Dataset is unique.
If you don't use the same PK in the Database and in the EJB you won't get an error if you insert the dataset. I know that sounds a little bit strange but perhaps there are situations where it can be useful.
I think its strange behavior too.
turn it off :
This option seems to be added in jboss 3.2.x (as I can see in the DTD)