Why is getShipmentList firing 'for update' queries on entries from yfs_order_header?

Question

Why is getShipmentList API firing a number of "for update" queries on entries from yfs_order_header? As this is  a list API, the expectation is that the API will not lock any records.

Consider the below input passed to the API -

<Shipment DeliveryMethod="PICK" EnterpriseCode="DEFAULT"
MaximumRecords="5000" SellerOrganizationCode="DEFEULT" ShipNode="Node1">
<ComplexQuery Operator="And">
<Or>
<Exp Name="Status" Value="1100.70.06.10"/>
<Exp Name="Status" Value="1100.70.06.20"/>
</Or>
</ComplexQuery>
<OrderBy>
<Attribute Desc="N" Name="Createts"/>
</OrderBy>
</Shipment>

The template being used is

<Shipments TotalNumberOfRecords="">
    <Shipment AssignedToUserId="" DeliveryMethod=""
        ExpectedDeliveryDate="" ExpectedShipmentDate="" HoldLocation=""
        OrderHeaderKey="" OrderNo="" RequestedShipmentDate=""
        ShipNode="" ShipmentKey="" ShipmentNo="" Status="" StatusDate=""
         TotalNumOfPickableSKUs="">
        <ShipmentLines>
            <ShipmentLine OrderNo="" >
                <OrderLine InvoicedQuantity="" OrderHeaderKey=""
                    OrderLineKey="" OrderedQty="" OriginalOrderedQty="">
                    <PersonInfoMarkFor FirstName="" LastName=""/>
                    <PersonInfoShipTo FirstName="" LastName=""/>
                </OrderLine>
            </ShipmentLine>
        </ShipmentLines>
        <BillToAddress AddressLine1="" AddressLine2="" AddressLine3=""
            City="" DayPhone="" EMailID="" EveningPhone="" FirstName=""
            LastName="" MobilePhone="" State="" ZipCode=""/>
        <Status Description="" ProcessTypeKey="" Status="" StatusKey="" StatusName=""/>
    </Shipment>
</Shipments>

This results in a for update query being fired on all the order header keys retrieved by the API, causing row lock contention.