... Kirby was just an employee.
Actually, I was under the impression that Kirby was even less than an employee, or at least, less than a full-time or contracted employee; his services were engaged on a freelance basis only for most of his career. But I think I get what you're saying anyway.
The problem is that no one at Marvel or DC could have anticipated the long-term financial windfall that would result from Kirby's work, or Seigel and Shuster's. The work agreements were made under the assumption that the financial returns would be modest, if there were any profits made at all. The outsized, runaway success of the creative properties took the companies and the creators by total surprise, and neither side made provisions in advance to set up a fair distribution of such ginormous profits.
It's pretty standard, I think, for parties to request renegotiations of work agreements when the resulting profits are much larger than the parties expected when the original agreement was made. In the Kirby and S/S cases, the profits weren't just large, they were mega-lottery sized, and they kept growing and growing and growing, with no end to the growth in sight, even all these years later.
I think it is a question of ethics, meaning simply that it's a question of fairness: if you create something that becomes profitable beyond your or anyone else's wildest expectations, is it fair for you to expect a significant chunk of the profits, more than you would have gotten if the payout had been normal-sized?
In the U.S., we assume that in those rare instances where someone's idea turns the whole world upside-down, the people who benefit financially from that idea will renegotiate if necessary so that all the parties that have a reasonable claim to the profits get a reasonable share. Our legal system exists to guarantee an equitable outcome for both employees and employers in cases where one tries to rape the other or otherwise withhold reasonable compensation.
What's "reasonable" in the Kirby or S/S cases? Obviously, people disagree strongly about that. What I find distressing and somewhat befuddling is the impulse on the part of many comics fans to side with the faceless, unfeeling, abstract corporations in these cases, rather than with the individual human beings who have reasonable-sounding grievances.
However, it is nice to hear those fans' viewpoints defended and explained, frankly, because I don't hold those views, but I would very much like to understand why others do.
(Similarly, I find it very hard to understand why anyone who knows what it's like to care about something you've created would fail to sympathize with Alan Moore. Whatever you think about his sanity or lack thereof, his ethics are beyond unimpeachable, from what I can tell; I mean, according to several outside observers, the guy has legitimate grounds to bring a giant lawsuit against DC/Warner Bros. for breach of contract, yet he has chosen not to do that for personal reasons. In other words, this is a man who is willing to turn his back on millions of dollars for the sake of his abstract principles, and he's done so on more than one occasion. I don't know if I could stand to do the same, in his shoes, but I certainly respect and admire him for doing it.)