Looks like you've discovered a bug - or is it a feature?
I checked it too on JBoss 5.1 - got the same results as you. JBoss for some reason allows this specification violation concerning extended contexts.
Glassfish is the reference implementation of all
Java EE specs, so maybe its best to practice SCBCD stuff with Glassfish.
Still, I find it very disappointed to find out that a supposedly Java EE certified application server allows this sort of spec violation.
Maybe its worth to check the JBoss bug list to see if its a known issue. Next I think I'll try JBoss 6 CR1 to see if it behaves the same way. I'll post my results here