Entity beans and transactions

[Vipin Mohan]

hi friends
I have some questions regarding the transactional behavior of entity beans.
1. Why do the specs specify that entity beans cannot support bean managed transactions ? I can understand it in the case of CMP beans. But in the case of BMP beans, why can't the programmer be allowed to manage the transactions ?
2. I read that for portable entity beans , only the Required, RequiresNew, or Mandatory transaction attributes must be used. What is the logic behind this rule ?
Thanks
Vipin