FK check implementation

I'm looking at the code behind the foreign key checks in ri_triggers.c, and
something's got me a little confused.

In both cases (FK insert/update checking the PK, and PK update/delete
checking the FK) the check is done with a SELECT ... FOR KEY SHARE.

This makes perfect sense for PK checks, but in the FK check, it seems
pointless at best; if it actually manages to find something to lock, it
will fail the check and error out moments later. And in any case, I don't
see how the key fields in the FK relation (to which the KEY SHARE lock
applies) are even relevant to the constraint in question.

What am I missing?