In the past (EJB 1.1), EB can be implemented by either BMP and CMP. At that time, as the containers were not good enough, and not that reliable, people tend to use BMP, which manage transactions by themselves.
Nowadays, as the containers improved a lot, CMP will be more reliable, and the containers will manage all transactions with DB. Thus, in EJB 2.0 specification, BMP is no longer allowed in ET becos the transaction services is provided by the containers, and they really did a cool management job on this area. BMP is still allow for SB, but usually, people will reliable on the containers' services.