In general, this is indeed in order to maintain backward compatibility. Note that the compatibility must be kept on the bytecode level too, and changing the return type changes the bytecode. So in general, if there are any subclasses which may have overridden the method in question, switching to a covariant return type would break those classes.
Since Graphics2D
is abstract, it is obviously meant to be subclassed, so the above reasoning applies.
Java Generics and Collections, although focuses more on the generics point of view, contains a discussion on covariant overriding in section 8.4.