I agree completely in your principal but the cruise line, the officers and the crew do not want other passengers to escalate a situation. A cruise ship is not an open society and not the streets of the city.
I find nothing more loathsome than seeing some cretin treating someone, who's job it is to offer them comfort and service, like a second class human being. The people who do this are nothing more than bullies or have their own issues to deal with.
The problem is that many people will intervene with the most noble intentions and very little tact and the person who ultimately suffers is the crewman. You may hear about how a bully or obnoxious passenger was put in their place by someone but you don't hear about the screaming matches, or worse, that ensue when two or more passengers get into it and a crew member now has a worse situation to deal with.
That is why I stated "find another crew member to help."
(Within reason: If someone is physically assaulting a crew member, then it's time to step in.)
What you say to the passenger, at a later time, is your own business.