The more check-boxes they add to the booking process, the more people will simply not book a cruise. That's why they recently changed the booking engine to automatically select guarantee staterooms. Fewer clicks, easier checkout process, fewer "abandoned carts".
I know every time I do a mock booking I get a call from my PVP later that day or the next asking if I need help.
While I think it would be great to allow folks to roll in Cheers etc to their cruise payments, I see why they don't.