The US government is applying severe sanctions against a US company that does not "influence their supply chain". Donald Trump believes the economy is great and at the same time declares economic emergencies to justify doing certain things. It could be true that Anthropic's products are useless for the DoD because of the products' safeguards, but that doesn't mean they're a risk to the US government.

As to this being 100% legal, I'm not so sure (not a lawyer). It might not be a criminal offese, but there's a whole category of abuse of power that this may fall under if Anthropic is put under a certain status without real justification. Many powers given to the executive branch are not absolute and can't be applied arbitrarily, but require justification. Anthropic might be able to sue the government for declaring them a "supply-chain risk" without sufficient justification. E.g. they could claim that not being sufficiently patriotic in the eyes of the administration does not constitute a risk, and that since their not the sole supplier of the tech, they were not trying to strong arm the government to do anything.