This annotation is a no-op (tested in Wicket and Restful Objects) and should be removed from the applib
... perhaps. Looking again at the code, there's a check in PersistenceSession#makePersistent() and throws exception if attempt to explicitly persist.
However, JDO might persist anyway due to persistence-by-reachability, so the check would need to be added to the appropriate JDO lifecycle callback.
As I think all the above is marginal benefit, think we should just remove in 2.0.0