I guess there is also a fundamental difference in business model across the pond - I have already splurged on the Haven or whatever suite product by virture of the (much) higher base fare. The fact a proprotion of that increased fare does not go towards the remuneration of the butler is a choice of the cruise line, and the line has an expectation the gap will be made up by the guest (you term it a responsbility, I might say 'guilt trip'), but I find it bizarre to operate this way, and it is clearly advantageous to the line. They do this with DSC too (a seperate debate I know).
It is fair to say 'when in Rome' - I find this way of business very tacky but, I am aware of it, therefore I would comply in say a US restaurant and tip 15-20% as would be customary, but I absolutely do not understand or agree with it. It disguises the true price amongst other things. But I know it's expected, and I do it.
However - in the butler example - his actions negatively affected my experience and therefore any 'base' tip I would give to comply with the 'guilt trip' was erased.