Why does index not use for CTE query?

Why does index not use for CTE query?
I am using Postgresql 9.1

select version();

version
--------------------------------------------------------------------------------------------------------
PostgreSQL 9.1.1 on i686-pc-linux-gnu, compiled by gcc (GCC) 4.1.2
20080704 (Red Hat 4.1.2-50), 32-bit
(1 row)

\d t1
Table "public.t1"
Column | Type | Modifiers
--------+---------+-----------
i | integer |
nam | text |
Indexes:
"t1_i_idx" btree (i)

analyze t1;
explain select * from t1 where i=2;
QUERY PLAN
--------------------------------------------------
Seq Scan on t1 (cost=0.00..1.09 rows=4 width=9)
Filter: (i = 2)
(2 rows)

set enable_seqscan = off;

explain select * from t1 where i=2;
QUERY PLAN
--------------------------------------------------------------------
Index Scan using t1_i_idx on t1 (cost=0.00..12.32 rows=4 width=9)
Index Cond: (i = 2)
(2 rows)

explain
with q as (select * from t1)
select * from q where i=2;

select * from q where i=2;
QUERY
PLAN
------------------------------------------------------------------------------
CTE Scan on q (cost=10000000001.07..10000000001.23 rows=1 width=36)
Filter: (i = 2)
CTE q
-> Seq Scan on t1 (cost=10000000000.00..10000000001.07 rows=7
width=9)
(4 rows)

Index is not using here.
Could you tell me why?