"for SELECT DISTINCT, ORDER BY expressions must appear in select list" - is that the standart or a limitation of postgresql?

CREATE TABLE test
(
id int4 NOT NULL DEFAULT nextval('teste_id_seq'::regclass),
name varchar,
number int4
)
WITHOUT OIDS;

The data:

1;"walter";1
2;"walter";1
3;"walter";1
4;"walter";1
5;"walter";2
6;"walter";3
7;"rodrigo";1
8;"rodrigo";2
9;"rodrigo";3

The query:

SELECT distinct name from test order by number

(well, I think that que query doesn't make any sense, but raises the error :) )

The error: ERROR: for SELECT DISTINCT, ORDER BY expressions must
appear in select list is due to a standart implementarion or a design
decision of postgres?

The comentary on parse_clause.c looks like the second option. I'm right?

[]'s
- Walter