I am aware of the rules, which is why I did not say they will get you to the ship at their cost. What I did say is that they will take care of dealing with the airlines to get you to the ship, although the cost is your obligation.
But there is still some benefit in that.
I never said or implied that buying air through a cruise line is any different, I just said that the cruise line (or travel agent, assuming you used a real one) will assist you in getting those tickets changed at your cost - still a worthwhile convenience if you are sitting in an airport.
And, some cruise lines do offer a premium service where you can change the flights without a penalty.
As far as I know, though, the top tier Princess air program is the same as Royal Caribbeans. Neither one will pay to get you to the cruise - but they will assist you in making those arrangements.