It's not the cruiseline's fault. "Casual" ACTUALLY means a suit, as opposed to a tux, if you want to get semantic about it.
But really it just comes down to two competing things:
1). The actual deffinitions
2). What people actually wear.
Smart casual means a jacket and a button-front shirt. I would argue a tie as well, but if the rest of your attire looks like you give a damn, you can probably skip the tie. Smart casual does NOT mean dockers and a polo shirt. It doesn't matter if every single male on the ship and half the females are in that attire, it is STILL wrong.
But the fact is, on every cruise ship, there will be people wearing dockers and polos to formal nights, sneakers, shorts when they can, and just thinking that everyone else is nuts for dressing up on vacation. (6 men without a jacket is simply 6 wrong men, not 6 men who know how to dress properly)
So do what you want, you will almost always manage to fit in. You will be WRONG, but you will almost always fit in.