Why can I insert into a column that is defined as NOT NULL?

try these . . .

create table t
(
    col1    int,
    col2    int not null
)

insert into t (col1, col2) select NULL, 2
/*
(1 row(s) affected)
*/

insert into t (col1, col2) select 1, NULL
/*
Server: Msg 515, Level 16, State 2, Line 1
Cannot insert the value NULL into column 'col2', table 'test.dbo.t'; column does not allow nulls. INSERT fails.
The statement has been terminated.
*/

drop table t