Subquery in the dbms_* procedures

Hello,

it is very useful to use subqueries in the procedures, and my 1st sentence is working:

select output from table( DBMS_WORKLOAD_REPOSITORY.AWR_REPORT_HTML( (select dbid from v$database), (select INSTANCE_NUMBER from v$instance), (select min(snap_id) from DBA_HIST_SNAPSHOT), (select max(snap_id) from DBA_HIST_SNAPSHOT)  ) ); / 

But 2nd isn't working:

exec DBMS_WORKLOAD_REPOSITORY.DROP_SNAPSHOT_RANGE( (select min(snap_id) from DBA_HIST_SNAPSHOT), (select max(snap_id) from DBA_HIST_SNAPSHOT) ); 

And I don't understand why 2nd produce the mistake, while 1st is working ?

Explain, please, is it bug or feature ?