SELECT FOR UPDATE/SHARE cannot be applied to the nullable side of an outer join

Given something like:

create table foo (id int primary key not null);
create table bar (id int primary key not null, a_id int references
foo(id));
select a.id, b.id from foo a left outer join bar b on (b.a_id =
a.id) for update;

PG 8.1.4 balks, saying:

SELECT FOR UPDATE/SHARE cannot be applied to the nullable side of an
outer join

Is this behavior spec-mandated, or could only the rows in B which are
not-null be locked?

----
James Robinson
Socialserve.com