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'CREATEtest=>insert into test (x) values (100);INSERT 19359 1test=> 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 ytest=>
insertinto test  values (100, 0);INSERT 19363 1test=> insert into test  values (100, 0);ERROR:  Cannot insert a
duplicatekey 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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