A mutually agreed upon "you may not use our product for X or Y purpose" in a contract clearly doesn't pose any such risk.
Imagine if Microsoft tried the same with excel or power point. "um actually in the fine print of the EULA it says you may not use our product for military purposes so we will revoke your access unless you stop using power point for mission briefings that might result in the deaths of civilians". Would fly about as well as a lead brick.
As a sanity check, I’d highly recommend trying out your own line of reasoning in cases which you think you might feel differently about. For example, imagine it's instead a certain authoritarian state saying "obey the Party or get blacklisted as a national security threat."
It might feel good in the moment, like "yeah, Anthropic, don't tell the military what to do," because you agree with this particular call. But the government doesn't give that power back, and you might not be so happy about the next way it gets used.