This feature was included under "Ideas explored and not standardized" in an appendix of the JSR-303/Bean Validation specification and simply wasn't ready for prime time at the time the spec was published. OpenJPA could have chosen to implement this feature, but at the risk of implementing a feature that most likely will be updated and possibly cause backward compatibility issues in a future version. It was decided not to implement the feature for that reason. My guess is that we'll see this feature become standardized (non-optional) and expanded upon in the next revisions of the Bean Validation and JPA specifications.
Side note - scanning for these annotations in OpenJPA would be fairly trivial. Processing constraints defined in XML will be more difficult. OpenJPA would need to either do all the same annotation + constraint XML processing as the Bean Validation provider or tie into whatever constraint metadata is supplied by the validation API to determine which constraints need to applied as mapping information. If we considered adding this feature to a future version of OpenJPA 2.x.x (based on JPA 2.0 and BV 1.0), my preference would be to require users to specifically enable it via an option on the configuration and mapping tool.