They certainly can restrict technology that is deemed to be a national security risk.
I guess that is how it always starts.
The fascist is inside the building.
Trying to discern the motive for something like this in a court is a losing game. The opposing side has a good argument ("Anthropic said this is a weapon, so we're controlling it like a weapon") that does not involve first amendment protection.
Anthropic shot themselves in the foot, and they have nobody to blame but themselves.
There are multiple models at the level of or better than Fable / Mythos (see OpenRouter's just released Fusion API) and none of them are getting restricted.