Casting Integer to Boolean in assignment

Hello the list,

Maybe this question has been debated before (I didn't find anything 
helpful) but :

Why the default is to throw an error when casting Integer to Boolean in 
assignment, and accepting it everywhere else ?

So you can type :

postgres=# select 1::boolean;
  bool
------
  t
(1 row)

or

postgres=# select 0::boolean;
  bool
------
  f
(1 row)


But you *cannot* use 1 or 0 as valid input for boolean type when 
inserting or updating :

test=# CREATE TABLE test1 (a boolean);
CREATE TABLE
test=# INSERT INTO test1 VALUES (1);
ERROR:  column "a" is of type boolean but expression is of type integer
LINE 1: INSERT INTO test1 VALUES (1);
                                   ^
HINT:  You will need to rewrite or cast the expression.


This behavior cannot be changed, as this cast is hard coded with 
"Implicit?=no".

And added to this weirdness is the fact that '1' or '0' (with quote) is OK.

So is there a reason to forbid 0 and 1 as valid boolean, without 
explicit cast ?

Thanks!