For the fallacy see here.
An American friend once told me that he wanted the United States to maximize its citizens’ utility. I asked, why just its citizens? Is there some moral reason to prefer Americans to foreigners?* No, he said, but obviously Americans would never vote to maximize global utility. Fine, I said, but why don’t you want to? Should we harvest African babies if it could increase American total utility? No, he admitted, we should probably adopt some formula that maximized the sum of American utility and some fraction of non-American utility – perhaps 1/2).
That struck me as odd for two reasons. My friend was implying that foreigners counted for a fraction of the moral worth of Americans – which he did not believe. And he was adopting a utility scale that a purely rational electorate would reject nearly as inevitably as it would reject a “global utility” formula. He had adopted it as a hybrid between what he probably preferred – equal weighting of utility – and what he considered inevitable – national maximizing. Given that he was capable of embracing an impossible principle, why didn’t he embrace the one he actually believed?
I should probably apologize for ambushing my friend at an irrelevant tangent in our conversation.
Sometimes the fallacy is blindingly obvious. If you ask someone for their opinion on abortion, they will give it to you. Suppose they are pro-life. If you tell them that they should change their opinion, because the Supreme Court has decided the issue, they will shrug. What does the Supreme Court’s opinion have to do with theirs?
I suspect that many people who consistently commit the fallacy of inevitability have delusions of politics: they want to be President someday. They are not eager to embrace obvious moral principles that are contrary to the interests of voters (like equal moral worth for foreigners). So they pretend to support things that are inevitable. This is one way that political incentives weed honest people out of politics.
* Nothing in this post should be construed to suggest I advocate maximizing global utility.