I really can't speak to the technical merits of JDO vs Exolab's implementation.
But Sun is extremely intolerant of anyone thinking that they have a better implementation of something of anything connected with Java.
The history of this pattern:
1. Java Generic Library -- Sun had to create the Collections Classes
2. Apache Xerces - Sun has create it's own XML doc handling
3. Apache LOG4J - Sun created its own, less flexible implementation that was based off of LOG4J and placed it into java.util.*
4. JUNIT - Sun couldnt STAND that, and placed assertions into the language itself. I personally dont think it adds to the language.
5. Now STRUTS - a great idea, great implementation, didnt cost Sun anything -- is now under attack with JFACES.
Instead of rationalizing the Java language, Sun just keeps throwing more kitchen sinks into it.