The risk to booking a bulkhead seat is being moved for a passenger who needs it because of a disability (or perhaps a bassinet for an infant in arms on a flight with bassinets). If I book a bulkhead and get moved for a passenger who needs it for a disability or an infant in arms, well that’s the risk I take. But I shouldn’t be moved from a bulkhead seat to accommodate a family who didn’t want to pay a fare that didn’t allow them to select seats in advance, especially if I have to otherwise pay extra for the bulkhead seat.