Inserting duplicate records in Primary key field

I had a mail from my friend describing the way to inserting duplicate value in a primary key field. I have reproduced it below:

First let us create a table:

create table test100
(
empno number(9) primary key,
ename varchar2(50)
);

Insert some valid records into the table:

insert into test100 values(1,'Sachin');

insert into test100 values(2,'Saurav');

commit;

EMPNO ENAME
1 Sachin
2 Saurav

Find out the constraint name for the primary key:
SELECT CONSTRAINT_NAME FROM USER_CONSTRAINTS WHERE TABLE_NAME LIKE 'TEST100' ;

CONSTRAINT_NAME
SYS_C00249950

Disable the constraint:

ALTER TABLE TEST100 DISABLE CONSTRAINT SYS_C00249950;

Check whether the index for the field is enabled or not:
select index_name,index_type from user_indexes where table_name like 'TEST100';

It should return no rows returned. If otherwise returned drop the index.

Now insert the duplicate record:

INSERT INTO TEST100 VALUES (1,'Ganguly');

Now create a non-unique index on the field:
CREATE INDEX TEST100_INDEX ON TEST100(EMPNO);

And at final enable the primary key constraint:
ALTER TABLE TEST100 ENABLE NOVALIDATE CONSTRAINT SYS_C00249950;

Now give a select * from test100;

EMPNO ENAME
1 Sachin
2 Saurav
1 Ganguly

Now you have a enabled primary key constraint with a violated data:
SELECT CONSTRAINT_NAME,STATUS FROM USER_CONSTRAINTS WHERE TABLE_NAME='TEST100';

CONSTRAINT_NAME STATUS
SYS_C00249950 ENABLED

Can anybody give suggestions as to why Oracle has given the NOVALIDATE clause while enabling the constraint?