It struck me too in the beginning and i reasoned as below. If someone knows better, then pls let us know
For BMT, the bean can use UserTransaction to start a transaction. So, it can call on resource manager's methods and other enterprise beans.
For CMT, probably the bean can call other enterprise beans which (probably) do not need a transaction and can manage their own transaction. I am not sure what the bean can do with Resource Managers.
Same for a Stateless session bean, it can call getUserTransaction in it's ejbCreate but cannot use any of its methods. So, it can just hold on to it. Its something like "being allowed a cake but it's just too soon to eat it".