I'm not saying it's not good business to keep sick passengers off a ship...but it's VERY poor business to not offer compensation. The passenger is not making the choice that they feel so poorly that they would chose to not travel. In addition, there is a medical facility onboard every ship.
I don't think the ship has the right to refuse without compensation. They can offer stipulations like quarentine a passenger to their cabin until medical release is received, etc but they propbably don't want to deal with that...so, they should offer compensation.
The difference between a ship and an airline is space. Other passengers are at the mercy of the sick traveler and do not have a choice. They are enclosed in an area that has no ventilation. That is not the case on a ship...and again, medical is available. I'd be curious what the legal stipulations are. Just because these are the cruiselines guidelines, it doesn't mean it's legal or just