why doesn't this work?

Conceptually I don't understand why this doesn't work.

from the psql prompt:

plumbing=# begin;
BEGIN
plumbing=# insert into plan_prices
plumbing-# (plan_id, eff_date, ground, topout, final)
plumbing-# values
plumbing-# (1, '2001-01-01',1000,1000,1000);
INSERT 215620 1
plumbing=# select currval('plan_prices_id_seq');
 currval
---------
      11
(1 row)

plumbing=# SELECT * from plan_prices ;
 id | plan_id |  eff_date  | ground  | topout  |  final
----+---------+------------+---------+---------+---------
  1 |       1 | 2000-01-01 | 1000.00 | 2400.00 | 1700.00
  4 |       1 | 2000-06-01 |    0.00 |    0.00 |    0.00
  7 |       1 | 2001-01-01 | 1000.00 | 1000.00 | 1000.00
 11 |       1 | 2001-01-01 | 1000.00 | 1000.00 | 1000.00
(4 rows)

plumbing=# delete from plan_prices where id=11;
ERROR:  triggered data change violation on relation "plan_prices"
plumbing=# ABORT ;
ROLLBACK

--
shaky cellar