Adding unique constraint with enable validate locks table?

TABLE: 128  Partition by Hash (LNAME) ROWS: 300 Million rows
DB: Oracle 10.2.0.4
RAC DB: 5 nodes on Linux


Question:
when I run the following statement to add constraint with enable validate,wHY does this add constraint statment block the simple select on this PROD.T_PERSON table,. Is there a Oracle doc that explain this behavior.

select * from PROD.T_PERSON;

*****************************************************

BUILDING CONSTRAINT with ENABLE VALIDATE:

alter table PROD.T_PERSON add CONSTRAINT U_PERSON2 
UNIQUE (CID, SYSTEMCODE) DEFERRABLE INITIALLY DEFERRED 
USING INDEX PROD.U_PERSON2 ENABLE VALIDATE;


 UNIQUE KEY INDEX:
CREATE INDEX  PROD.U_PERSON2 ON  PROD.T_PERSON
(CID, SYSTEMCODE)
NOLOGGING