SeqScan vs. IndexScan

Hi,

I'm running the same query with "set enable_seqscan = on;" and "set 
enable_seqscan = off;":

->  Nested Loop Left Join  (cost=0.00..89642.86 rows=1 width=30) (actual 
time=1.612..6924.232 rows=3289 loops=1)
       Join Filter: (sys_user.user_id = j_6634.id)
       Rows Removed by Join Filter: 14330174
       ->  Seq Scan on sys_user  (cost=0.00..89449.85 rows=1 width=16) 
(actual time=0.117..39.802 rows=3289 loops=1)
             Filter: ...
       ->  Seq Scan on cmn_user j_6634  (cost=0.00..138.56 rows=4356 
width=22) (actual time=0.001..0.973 rows=4358 loops=3289)

(Full plan: https://explain.depesz.com/s/plAO)

->  Nested Loop Left Join  (cost=0.56..89643.52 rows=1 width=30) (actual 
time=0.589..39.674 rows=3288 loops=1)
       ->  Index Scan using sys_user_pkey on sys_user 
(cost=0.28..89635.21 rows=1 width=16) (actual time=0.542..29.435 
rows=3288 loops=1)
             Filter: ...
       ->  Index Scan using cmn_user_pkey on cmn_user j_6634 
(cost=0.28..8.30 rows=1 width=22) (actual time=0.002..0.002 rows=1 
loops=3288)
             Index Cond: (sys_user.user_id = id)

(Full plan: https://explain.depesz.com/s/4QXy)

Why optimizer is choosing SeqScan (on cmn_user) in the first query, 
instead of an IndexScan, despite of SeqScan being more costly?

Regards,
Vitaliy