Foreign keys breaks tables permissions

Hi,

If one does:

 create table master (
  id integer not null,
  primary key (id)
 );

 create table detail (
  id integer not null,
  master_id integer not null,
  primary key (id),
  foreign key (master_id) references master (id)
 );

 insert into master (id) values (1);

 grant select on master to a_user;
 grant select, insert, update, delete on detail to a_user;

then if login as "a_user" and does:

  insert into detail (id, master_id) values (1, 10);

this will result in: "ERROR:   master: Permission denied".

This seems a bug to me ? Isn't it ?

Regards,
Raul Chirea.