It actually is pretty easy morally, depending on what your moral is ;-) It is easy because to get into this tax bracket, you have to earn a LOT of money, way way more than the average citizen. If you hit this, you still have at least 3x net as much as everyone else. There is no reason to be sorry of someone having to contribute loads to the public and still being super well off afterwards. (Having lived in both, this is actually a huge cultural difference between Europe and the US)

The old question is whether this results in less motivation to do work which brings you above the (whatever) top tax bracket. In my experience it actually is.

Counterpoint: is it moral for a small minority of citizens to have, say 1000x the wealth (and therefore the power) of the average citizen, and therefore have a disproportionate amount of political influence, and therefore being able to force upon the general population, laws and policies that they did not vote for?

Also, I do understand your point of view. When I was an ardent Ayn Rand fan (much younger then), I used to use this same argument, an in fact the same phrasing: taking income at the threat of violence (which, IIRC, is from Capitalism: The Unknown Ideal). I've just come to realize over the years that the opposing viewpoint is actually morally superior.

I'd agree that Ayn Rand is a step too far. I don't think the pure libertarian mind set is at all practical. But their arguments do have weight when you start to get to this level of taxation.

I see no issues with some having 1000x the wealth of others. We both probably have 1000x the average citizen in some of the poorest countries in the world. Is this a problem yes. Are we morally wrong for having that wealth, no I don't think so.

Is there a problem with the difference in power at the political level, yes, is this a moral issue yes. But I don't think taking from the rich giving to the poor solves this problem. The reason being that as we do this it tends to make the situation worse. Welfare programs tend to restrict the freedom of those on the programs more that the rest of the population and have been shown to develop a learned dependence on these programs. Even if it did work I don't see how that justifies taking so much from someone.

It's a good idea to try and level the playing field. But I don't think massive income taxes are moral and as such should not be chosen as the tool to do this.