I guess because technically nothing says WHEN you have to ascertain if an assignment has been placed on your schedule. In theory you could wait until midnight or later once you start LC to view your schedule (or the daily trip coverage, or the daily SC assignment log, or text your buddy to view your schedule, etc). While in practice most people check fairly soon after the 12 hour prior point, since that is voluntary at that time it isn’t considered to have broken your rest.