Serial and NULL values

I just received a message from someone complaining about SERIAL/sequence. I
think there is a problem:

test=> create table test (x int, y serial);
NOTICE: CREATE TABLE will create implicit sequence 'test_y_seq' for SERIAL column 'test.y'
NOTICE: CREATE TABLE/UNIQUE will create implicit index 'test_y_key' for table 'test'
CREATE
test=> insert into test (x) values (100);
INSERT 19359 1
test=> insert into test (x) values (100);
INSERT 19360 1

These work fine, but why does this fail:

test=> insert into test values (100, null);
ERROR: ExecAppend: Fail to add null value in not null attribute y
test=> insert into test values (100, 0);
INSERT 19363 1
test=> insert into test values (100, 0);
ERROR: Cannot insert a duplicate key into a unique index

Can't they use zero or null, and have the sequence value be computed?
Is there some design decision we made to prevent this?

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