The cynical answer would be: look at the membership:
OASIS Web Services Business Process Execution Language (WSBPEL) TC Do you think
Microsoft is going to let
Java technology become part of the standard?
In truth WSBPEL is targeted as an interoperable specification - letting any platform specific technology into the standard would defeat that objective. However the Java EE vendors are free to define their own extensions - which apparently IBM has done. In fact Sun Microsystems could make a BPEL-EJB extension part of a Java EE profile that vendors could choose to implement.
Whether or not a BPEL-EJB extension makes sense is another matter. It may seem that EJBs are on an equal footing with web services because starting with J2EE 1.4 you can turn stateless sessions beans into web service endpoints. However a web service is typically an entire service
application while an SLSB is usually a mere
transaction script. The distinction isn't that clear-cut but web service end points should in general support operations of a larger granularity than what is typically supported by EJBs.