The forkers have established - perhaps quite reasonably - the principle that Ethereum isn't a set of rules unto itself, but it includes the ability to right what are externally perceived as wrongs even when they are done by playing according to the internal rules. So the forkers have set a precedent for more forks, as needed, whenever the community as a whole decides some sufficiently impactful party is cheating the spirit, rather than the letter of the law.

Except that the whole value prop of Ethereum was supposed to be that the letter of the law was all, and the system would unconditionally and inviolably enforce it.

Yes, exactly. That got, semi-explicitly, changed in the fork. While remaining so in the nonfork.