Why does the behavior of array syntax differ from '(?,?)' syntax when updating a point field and that field is NULL?

I’m using PostgreSQL 9.3.5.

Suppose I have the following table:

CREATE TEMPORARY TABLE point_test ("name" varchar(255), "pt" point);

I then insert a row, leaving pt NULL:

INSERT INTO point_test (name) VALUES ('me');

Then, I want to update pt using the array-like syntax:

UPDATE point_test SET pt[0] = 42, pt[1] = -42 WHERE name = 'me';

which appears to succeed (UPDATE 1) – EXPLAIN VERBOSE shows the following:

Update on pg_temp_65.point_test  (cost=0.00..11.75 rows=1 width=538)
  ->  Seq Scan on pg_temp_65.point_test  (cost=0.00..11.75 rows=1 width=538)
        Output: name, (pt[0] := 42::double precision)[1] := (-42)::double precision, ctid
        Filter: ((point_test.name)::text = 'me'::text)

However, pt is still NULL. If I use a slightly different syntax, it works in this case:

UPDATE point_test SET pt = '(42,-42)' WHERE name = 'me';

results in the point (42,-42) as expected.

Further, now that there is something in the field, I can update the point using the first syntax:

UPDATE point_test SET pt[0] = 84, pt[1] = -84 WHERE name = 'me';

results in the point (84,-84) as expected.

Why do the behaviors of the two syntaxes differ only when the pt column is NULL?