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)