Dear True Brit,
I suggest you stay there. Part of their salary they are promised when they sign on to work 16 hours a day 7 days a week for a min of 6 mo. is those tips.
They do exactly what work the cruise line instructs them to do, they are not doing extra cleaning in the rooms or turning down the bed to try to wangle extra tips.
I everyone thought like you and did not tip because they are just doing their job then you would quickly have no one serving you at all on the ship you could clean your own cabin serve all your own meals etc. or pay a heck of a lot more for your cruise fare.
and I doubt any of the posters in this thread work for cruiselines.