Morality as a concept is changing: some things become moral, some - immoral.
No it's not. There's what's right and what's wrong. When slavery was around there was a small amount of people around who knew it was wrong. The rest of society either didn't know because they hadn't thought about it or used the excuse that ii had been around a long time, or that the majority approved or that it was necessary or some combination of those. Unfortunately, that's all it was, an excuse. Same is true for today for taxation. Either people haven't thought about it or they're making some excuse for it because they either benefit or they don't have the imagination to think of life without it. But it's just an excuse, because there are people around pointing out the immorality of it just like there was back in the heyday of slavery.
And the same is true of human sacrifice or any number of accepted immoral traditions throughout history.
Man started in this world not knowing much and built all these traditions upon bad information. We now live in a more enlightened age. It's time that we were brave enough to recognise what is immoral rather than kowtowing to something just because the majority is currently going along with it. But history shows that only a minority stand up for what's right whereas the majority just go along with whatever they are told.
In other words, you're saying that Americans "didn't know" that slavery was wrong, but found out during the Civil War? The Confederate States, after having textbooks & sound reasoning hurled at them by the Union Army, completed their learnings, realizing how wrong slavery & their "screw you guys, I'm going home" attitude were, freed the slaves & embraced the Union? And, just like with slavery, we simply need to educate the great unwashed on the evols of taxation, and taxes will disappear?
There always were, and always will be, people pointing out the immorality of just about everything -- from atheism to unwed sex to lending for profit to homosexuality to taxation. None of that implies anything, certainly not that those opinions are valid, or that they will someday become cannon.