odd planner choice

I've run into this odd planner choice which I don't quite understand.

I have two tables articles, users and
articles.article_id and users.user_id are primary keys.

Insides articles there are two optional fields author_id1, author_id2
which all reference users.user_id.

And now the plans:
(by the way this is pg 7.4 and I set enable_seqscan to off).

jargol=# explain select user_id, first_names, last_name from articles, users
where article_id = 5027 and (articles.author_id1 = users.user_id);
                                    QUERY PLAN
----------------------------------------------------------------------------
------
 Nested Loop  (cost=0.00..4.04 rows=1 width=26)
   ->  Index Scan using articles_pk on articles  (cost=0.00..2.01 rows=1
width=4)
         Index Cond: (article_id = 5027)
   ->  Index Scan using users_pk on users  (cost=0.00..2.01 rows=1 width=26)
         Index Cond: ("outer".author_id1 = users.user_id)
(5 rows)

jargol=# explain select user_id, first_names, last_name from articles, users
where article_id = 5027 and (articles.author_id1 = users.user_id or
articles.author_id2 = users.user_id);
                                            QUERY PLAN
----------------------------------------------------------------------------
-----------------------
 Nested Loop  (cost=100000000.00..100000003.11 rows=2 width=26)
   Join Filter: (("outer".author_id1 = "inner".user_id) OR
("outer".author_id2 = "inner".user_id))
   ->  Index Scan using articles_pk on articles  (cost=0.00..2.01 rows=1
width=8)
         Index Cond: (article_id = 5027)
   ->  Seq Scan on users  (cost=100000000.00..100000001.04 rows=4 width=26)
(5 rows)

Why does it think it MUST do a seq-scan in the second case? users.user_id is
a primary key,
so shouldn't it behave exactly as in the first case?

Any enlightenment on this problem will be much appreciated.

thanks,
Ara Anjargolian