There is a fundamental difference between tipping in the USA and the rest of the world. In the US wait staff are paid a pittance, whilst in most other places in the world staff are paid a proper living wage. I live near Geneva, Switzerland and the minimum legal wage is about 25 CHF/hour (about 27 USD). I know Switzerland is a high cost country, but that's nevertheless orders of magnitude above what happens in the US. Wait staff are paid a proper living wage here and that's even true in France, where the minimum wage (the SMIC) is lower, but still just about allows you to live.
I understand the situation in the US. I've worked with many US colleagues who went through college and partly financed themselves by working and living off tips, because that was what was expected.
However, the tipping culture in the rest of the world is very different. I feel no need to tip waiters here, though I will usually round up a little, but certainly nowhere near the 20% expected in the US.
As far as Oceania is concerned, I will let the auto-tip/auto-service or whatever they call it, stand. I would prefer if it was built into the fares that we have to pay, but I recognize that most passengers are North American, that Oceania is a US company, and I therefore accept that's the way you have to do things. I hesitate about adding any more, sounds like tipping the tips.