If you use CMT, then @TransactionAttribute(TransactionAttributeType.REQUIRED)
would tell the container to check for an existing transaction and open one if there is none.
But if you use BMT, then it's your responsibility to do such a thing, so there's no one to observe the annotation above. Since it is still syntactically correct and the class is available, there is no need for the JVM to complain about.
Concerning ignoring annotations, there's a hint in the answer to this question.