This is the wrong level of abstraction to think about the problem at. You're talking about how it was done, in terms of technical implementation. Focus on the effects, in terms of real-world exchange value -- that's what matters, both to the law and to human beings.

In my analogy, Casey owned 1/3 of X (a corporation), where X is some social construct with real-world market value in terms of dollars or bitcoin or gold or whatever. The other owners of X worked to transfer real-world market value from X to a new social construct, Y, in order to capture Casey's share of the value. Under the US-law ruleset this is a violation of their fiduciary duty to Casey as a minority shareholder.

In the Ethereum example, the DAO hacker owned 1/20th (or so?) of X, where X (ether) is some social construct with real-world market value in terms of dollars or bitcoin or gold or whatever. The other owners of X worked to transfer real-world market value from X to a new social construct, Y, in order to capture the DAO hacker's share of the value. Under the Ethereum ruleset this is OK.

So we have two different rulesets with different protection for minority stakeholders. It's an interesting question whether (despite this example) Ethereum could end up being more favorable for minority stakeholders overall. But it's not at all interesting to spend time arguing about whether a minority stakeholder was injured by the Ethereum split -- that was the entire, explicit, point of the split, and if you focus on real-world exchange value there's no way to disguise it.