Yes, I saw the rule once deep in the recesses of Celebrity's website. I believe it's an industry-wide rule. Maybe somebody knows exactly where to pinpoint the reference on the website.
One of the few real dangers at sea (at least these days) is fire. Inadvertently leaving an iron plugged in and having it set a blaze is frightening. I suspect that the cruise lines are also thinking of the ships' power capacity. Irons pull a lot of juice, and I don't know what sort of hiccup the power system would have if everybody plugged one in a half hour before dinner.
In the past, strings about irons have triggered all sorts of indignant complaints to the effect that if irons are a fire hazard, then so are things like other electrical gizmos, smoking and maybe even sex. Me, I just can't get into the controversy. I said on another post recently that if somethings got a wrinkle, ignore it. Everybody else will. If somebody takes a photo of you and the wrinkle shows up, have the photo digitally altered when you get home.