plpgsql - or operator?

If I have an expression (a or b)a where a=TRUE and b=FALSE, why is b
evaluated? Any true operand before an or operator means the entire
expression is true .

---------------------------- EXAMPLE

create or replace function pinsusers() returns trigger as

$$

declare msg varchar;

begin

--THROWS EXCEPTION when ((TG_OP = 'INSERT') is TRUE "record old
is not assigned yet"

msg = ((TG_OP = 'INSERT') or (new.password<>old.password));

raise exception '%',msg;

return new;

end

$$ language plpgsql;

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TY :-)