Good catch :-)
Both of them might look alike, but they are quite different.
For example, Nested transactions can operate concurrently, where as the "REQUIRES NEW" stuff will make the callee to wait until the spawned transaction returns back!
EJB Spec (page no. 332) makes clear why nested transactions are not supported.
Note: The decision not to support nested transactions allows vendors of existing transaction
processing and database management systems to incorporate support for Enterprise Java-
Beans. If these vendors provide support for nested transactions in the future, Enterprise Java-
Beans may be enhanced to take advantage of nested transactions.