Hash Join not using hashed index?

I'm using Postgresql 7.02.

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# explain select city.name, country.name from country, city
where city.country_id = country.country_id;
NOTICE:  QUERY PLAN:
Hash Join  (cost=8.85..16.76 rows=75 width=18) ->  Seq Scan on city  (cost=0.00..1.75 rows=75 width=16) ->  Hash
(cost=5.53..5.53rows=253 width=2)       ->  Seq Scan on country  (cost=0.00..5.53 rows=253 width=2)         
 

EXPLAIN 
# create index country_id_idx on country using hash (country_id);
CREATE
# vacuum analyze;
VACUUM
# explain select city.name, country.name from country, city
where city.country_id = country.country_id;
NOTICE:  QUERY PLAN:
Hash Join  (cost=8.85..16.76 rows=75 width=18) ->  Seq Scan on city  (cost=0.00..1.75 rows=75 width=16) ->  Hash
(cost=5.53..5.53rows=253 width=2)       ->  Seq Scan on country  (cost=0.00..5.53 rows=253 width=2)
 
EXPLAIN
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Why doesn't PostgreSQL use country_id_idx, but rather rehashing 
country_id?