you just answered your own question. The game was not designed to run on an ARM architecture so even if it uses a Java VM it still wont function.
you can get Java 6 for the RPi through one of the repos.
looks like minecraft has the same problem
http://www.raspberrypi.org/phpBB3/viewtopic.php?p=190363 Ah, so it will get worked out eventually. If it gets to the point where minecraft has a solution then it may not be much trouble to add to jMonkeyEngine also. Then it will "just work".