If I committed to using jRuby instead of regular ruby, I would have that nagging fear that Oracle would go after all the other companies that build their product using Java APIs.
We work closely with engineers at Oracle and JRuby is one of the premier projects on the JVM. There's also no legal way they could come after us; their suit against Google is about copying Java APIs, a very narrow domain.
How would that help them? It doesn't seem like they have any incentive to prevent more people from using the JVM. In fact, I would imagine they want more people to use it, and multiple languages compiling to it makes it more popular.