Chef Ken - I am sorry but not allowing certain nationalities to take part, in free of charge, programmes aimed at rewarding faithful repeat clients IS discriminating!
To add insult very often these countries where cruising may not be as popular as in the US get hit a second time as cruiselines take a cut off the foreign exchange too.
Lets take cruisers from the UK for example. The GBP hit 2 USD today... Now you are paying 1.000 USD for a cruise, a cruiser from the UK won't be paying 500 GBP but rather 600 maybe even 800 GBP for exactly the same cabin!!
Now our poster from downunder gets nailed even worse - not only does he get to pay a premium, he has to fly forever to even get to the cruise but he doesn't get the little things you do as member of the Repeaters Club.
It may be legal but I bet it would upset you too if it applied to you.
Cruiselines are looking to expand their markets so they need to look at their incentives too.