select oid .... for update ....

 Here's something I don't understand....  If I'm missing an obvious, please
feel free to kick me in the right direction.
 We're given two tables, linked with a foreign key. When insert is run on a
master table, everything is OK, when  trying to insert into a detail table,
a strange query appears in the log (schema and log snippet attached).
 In fact, there should  be no problem, but if a user  has no permissions to
update the master  table (eg, this is  a log where he can  only insert, but
neither  delete, nor  update), then  inserting into  detail table  fails on
permission violation at the query:

SELECT oid FROM "alpha" WHERE "id" = $1 FOR UPDATE OF "alpha"

 TIA

 Ed


---
 Well I tried to be meek
  And I have tried to be mild
 But I spat like a woman
  And I sulked like a child
 I have lived behind the walls
  That have made me alone
 Striven for peace
  Which I never have known

 Dire Straits, Brothers In Arms, The Man's Too Strong (Knopfler)