Nothing in the spec constrains the provider to be capable of vending up any particular arbitrary implementation of Calendar you want, so it would be a compatibility issue if they allowed you to require it to give a particular one. (Not that anyone in their right mind would make a new subclass of Calendar, but the possibility is there. A JPA implementation that only knew how to return its own custom subclass of Calendar wouldn't really be wrong by anything in the spec, you can't require it to know how to use GregorianCalendar instead.)
Or on the flip side, if I were to create org.affe.MyAwesomeCalendar, it is not reasonable to expect a JPA provider to be able to make instances of it for me!
11.1.47 Temporal Annotation
The Temporal annotation must be specified for persistent fields or properties of type java.util.Date and java.util.Calendar. It may only be specified for fields or properties of these types.
Hibernate will not let you choose java.sql.Date etc directly either. Basically they interpret it's being left up to the persistence provider to determine what implementation of Calendar is used.