For me, when it specifically comes to copying, I don't think it's bad to copy a copier. (And by that I mean Anthropic has no valid complaints against Moonshot. Any valid complaints from anyone in the original corpus are valid against both of them now.)
In this way, it is different from literal theft. Stealing money/objects from a thief and keeping them is not justified.
It's a little different in this case, since 1) not all the data Ant used was stolen and 2) they did contribute significantly to the value of the stolen good.
An analogy might be a baker stole 20% of the flour used to bake their special bread, which was then stolen. Both thefts are obviously wrong and bad.
The exact same argument could be made in Moonshot's favor.
Being a bit tongue in cheek, one could also argue that by releasing their models, Moonshot is contributing much more value than Anthrophic. Did Prometheus not create an immense amount of value, when he took fire from the hands of the Gods and gave it to humans?
I think any analogy with physical theft is too different from data to apply to this comparatively subtle case. Especially when we get into the details of just using the output of the model to train on.
The baker stole 100% of the flour to make the bread. He also stole the water and the salt and the yeast and the heat for his oven. What he didn't steal was the time he put into crafting a recipe for bread and the time he sat around waiting for the oven to bake it. Now, is that loaf stolen property? Hard to say. But the baker is undoubtedly a thief. He should be tried and forced to pay restitution out of his ill-gotten profits for sure. If we can't do that, the next step is pitchforks and guillotines.
In this way, it is different from literal theft. Stealing money/objects from a thief and keeping them is not justified.