Shampoo is provided, however I guess that most cruisers prefer to bring their own brand. People have different likes and dislikes when it comes to shampoo.
As for irons, that is a big no no. The reason stated is always safety, but I will throw some gasoline on the fire and say that I believe that the real reason has nothing to do with fire at all. (When is the last time that you read about an iron causing a house fire? Probably never...)
I think the real reason that cruise lines frown on irons is the power they draw from the ship's main electrical system (generators). Image all the power that would be required the couple hours before formal night if most of the cabins had irons on. The generators would probably stall
05/89 NCL, Seaward, 1990 Carnival, Topicale, 1991 Carnival, Festivale, 11/93 Wind Star, Wind Song, 01/95 Celebrity, Horizon, 01/96 Celebrity, Century, 01/00 Celebrity, Zenith, 01/01 Princess, Ocean Princess, 01/02 Princess, Ocean Princess, 01/03 Celebrity, Summit, 05/04 Princess, Diamond, 01/05 Celebrity, Galaxy, 01/07 Oceania, Regatta, 01/08 RCCL, Voyager of the Seas, 02/09 RCCL, Voyager of the Seas, 01/10 Celebrity, Solstice