SQL & PL/SQL

I am currently toying with a union query whose members contain a cursor, and I get an ORA-22902 error.

Here is the query :

 select owner "obj_owner"
          ,table_name "obj_name"
	  ,'TABLE' "obj_type"
	  ,cursor(select owner
                     from all_synonyms
                     where table_owner like all_tables.owner
                     and table_name like all_tables.table_name
                 ) "syn_owner"
 from all_tables 
 where owner in ('USERA','USERB','USERC')
 union
 select owner "obj_owner"
          ,object_name "obj_name"
	  ,'PACKAGE' "obj_type"
	  ,cursor(select owner
                     from all_synonyms
                     where table_owner like all_objects.owner
                        and table_name like all_objects.object_name
                 ) "syn_owner"
 from all_objects
 where owner in ('USERA','USERB','USERC')
   and object_type like 'PACKAGE'
 order by "obj_owner" asc, "obj_name" asc
 ;

And here is the detailed error message :

ORA-22902 CURSOR expression not allowed
Cause: CURSOR on a subquery is allowed only in the top-level SELECT list of a query.
Action: Remove the CURSOR expression and retry the operation.

Now, I could easily get similar results otherwise, but I am curious as to why this query is erroneous, and I would like to know if it is possible to edit it to avoid this error.