grant user...HELP...

Hi,
i had create tables like this:
table one:
create table mk (
code_mk varchar(4),
nm_mk varchar(4),
sks int2,
primary key (code_mk));

table two:
create table thn (
th_ak int2,
tahun varchar(9),
primary key (th_ak));

table three:
create table nil (
numb varchar(8),
th_ak int2 references thn (th_ak),
code_mk varchar(8) references mk (code_mk),
grade int2,
primary key (numb,th_ak,code_mk));

then i grant my user with this command:
grant select,update on nil to jhon;
i want john able to select and update tuples on nil table, but when john try
to select tuples nothing error, but when he try to update tuples from nil
there's error that said he don't have permission on either mk or thn...
i don't want to grant john on mk or thn, is there any tricks to do this?
help...
other questions is when we grant update on some tables why does delete also
been granted?
when john try to delete tuples on nil there's no error?
and the references integrity on nil doesn't works?
please help...
TIA
Didi