I'm definately not one of the 80% who doesn't drink
It's not just alcoholic drinks where 15% is added to the bill - it's non alcoholic ones too. Were these people just drinking tap water i wonder? I think that in the UK tipping is a pretty alien thing and some people don't really know what to do. We've been to the USA many times so have had some practice.
It's true that the pre-paid gratuities are only for the waiters and stateroom attendents (on Royal Caribbean anyway) but we always give extra when we've had good service.