why it doesn't work? referential integrity

Hello

I found strange postgresql's behave. Can somebody explain it?

Regards
Pavel Stehule

CREATE TABLE users (
  id integer NOT NULL,
  name VARCHAR NOT NULL,
  PRIMARY KEY (id)
);

INSERT INTO users VALUES (1, 'Jozko');
INSERT INTO users VALUES (2, 'Ferko');
INSERT INTO users VALUES (3, 'Samko');

CREATE TABLE tasks (
  id integer NOT NULL,
  owner INT REFERENCES  users (id) ON UPDATE CASCADE ON DELETE SET NULL,
  worker INT REFERENCES users (id) ON UPDATE CASCADE ON DELETE SET NULL,
  checked_by INT REFERENCES users (id) ON UPDATE CASCADE ON DELETE SET NULL,
  PRIMARY KEY (id)
);
INSERT INTO tasks VALUES (1,1,NULL,NULL);
INSERT INTO tasks VALUES (2,2,2,NULL);
INSERT INTO tasks VALUES (3,3,3,3);

DELETE FROM users WHERE id = 1; -- works simple
DELETE FROM users WHERE id = 2; -- works ok
DELETE FROM users WHERE id = 3; -- doesn't work, why?

ERROR:  insert or update on table "tasks" violates foreign key
constraint "tasks_checked_by_fkey"
DETAIL:  Key (checked_by)=(3) is not present in table "users".
CONTEXT:  SQL statement "UPDATE ONLY "public"."tasks" SET "worker" =
NULL WHERE $1 OPERATOR(pg_catalog.=) "worker""