There are different rules for each country and because of that there are different prices.
Many cruise line booking systems will only allow internal agents or travel agents book clients in specific countries.
With travel agents, the selling agreements of the agencies have with the cruise line dictate what clients they can sell to.
For travel agents in the US, most cruise lines allow agents to book people who reside in US, Mexico, and Canada.
With trying to book a rate that you are not supposed to have, if the cruise line catches on they will make you pay the difference between what you paid and should have paid. Its not out of the realm of possibility that they deny you boarding as you were trying to commit fraud against the cruiseline.