I've seen this with other payment processors and I think it relates to chargeback risk. If a travel provider goes bankrupt or into administration (Bonza, Rex, Tigerair, Virgin Australia a few years back, etc etc) and you don't get what you paid for, a credit card chargeback is one recourse via your bank to recover your money. However, when you use any sort of intermediary payment processor, they themselves are the credit card merchant (it's why using third-party processors are popular for ATO payments, because it's not the ATO processing the transaction and so full points are earned). So, if they take your money for something intangible or a future service, the risk could be much higher because the card network can't take that stock back - the thing you bought just doesn't exist any more.
For a cruise, the issue could be that you pay a payment processor, which pays the cruise company. Let's say the cruise company cancels your cruise and refuses to refund, or goes bankrupt, etc etc. What are you going to do? You're going to want a credit card chargeback... only you didn't pay the cruise company, you paid some other business to pay your bill for you. They paid the cruise company by EFT, and you paid the processor. If you charge back your own initial transaction, you'd get your money back, cruise company keeps its money, and the payment processor is the one left making up the difference. I totally get why something like cruising would be a banned payment - the risk wouldn't be worth the tiny transaction margin.
