you can use other tools as well. I think the reason why HF selected that RI is because it's the simplest out there. It wants to focus on the basics of things, without having all those 'extra overheads that come with other tools' that could throw some extra tasks needed completion before you can see the result of the deployment.
During the learning stages, more time is spent in deploying the EJBs in a particular Container implementation than coding the EJBs. HF EJB uses SUN's Ref Impl b'cos it is easier and there is SUN's tutorial that could be of assistance also. The process of deployment would also differ from 1 App server to another.
His brain is the size of a cherry pit! About the size of this ad: