Are you saying that they are giving you first sitting dinner instead of a flexible time dining option? (Sorry...it's early in the morning and the tea hasn't kicked in yet.) I've usually heard of wait lists being the other way around.
If you have a set dining time and you know you won't be able to make it one evening, it is customary to let your waiter know in advance. We have done this for the reason you mentioned -- a shore excursion that overlaps dinner -- as well as for the one time per cruise we go to a specialty restaurant.
I haven't done flexible dining on RCI, but I've done it on HAL, and will do so again this summer. You can make a reservation (usually day-of), or just show up. There may be a wait if you arrive without a reservation, yet they may ask if you would like to dine with others, which can shorten the wait.