"I’ve never understood the “if I don’t enable bad behavior, someone else will, so I might as well enable bad behavior” argument. Can you elaborate?"
You are mentally approaching this as if you have an oracle that can be consulted to say whether or not something is bad behavior. So of course, if this oracle exists and can be consulted and it says the behavior is bad, why would anyone argue with the idea that we should stop bad behavior?
This argument is valid [1], in that give the premises the argument is correct. The problem is, once you draw out the fact that the argument is depending on the existence of an oracle that does not exist, that premise of the argument is invalid.
Two people can sit down in front of an AI right now, with the exact same code base, and type in a prompt to the AI "Analyze this code base for security holes and try to build exploits against them." One person's use is completely valid, another person's use is completely harmful, and the information necessary to distinguish those two use cases is not available to the AI. I phrase it that way carefully, it isn't that "the AI isn't smart enough", the problem is that the information is simply unavailable. Intelligence doesn't factor in at that point.
Therefore, the only way that Antropic has to deal with this at scale is simply to block the query entirely. Which means that when I, the valid user who is trying to establish whether my code base has security issues and whether I can prove they are exploitable, I can not. I am checking for exploitability because while I would like to fix all security issues, issues that are provable exploitable are of a higher priority than smelly code that doesn't seem to be exploitable, which is a perfectly valid thing for me to want to do.
If I can't use legitimate tools to secure my code, but the bad guys can use unrestricted tools to attack my code, now this is a great deal more complicated than "Who can argue with stopping the bad stuff?", which is the main point I want to make here. I'm not going into a huge analysis of that problem, merely pointing out that it is a problem and that this isn't just about "stopping the bad stuff". There are additional complications beyond that, like, even if Anthropic could determine the "bad stuff" and stop just that in their LLM, LLMs in general don't have infinitely precise surgical "stop doing this thing" options and any such instruction to stop doing a thing always degrades the LLM across the board in various ways.
Anthropic has no access to the Platonic ideal of "stop malware", if such a thing even hypothetically exists. When analyzing the real effects their real actions will take, what their intentions were for those actions aren't really relevant. It is clear that they are making their model a great deal less useful for me, a legitimate user, and I and others like me are perfectly justified in disagreeing with their analysis and actions.
I also observe that "the bad guys getting unrestricted access to the full power" is only a matter of time. There's no question whether it will happen, the only question is whether this time is in the past or the future. This includes the fact that while your definition and my definition of "bad guys" may vary, it is virtually certain that your definition includes at least one high-powered intelligence agency somewhere in the world that does cyberattacks and will have the means, the opportunity, and the motive to get unrestricted access to these models by means you may consider licit or illicit. If your threat model includes them, as mine does, it is perfectly reasonable to complain that my tooling is being broken in a ways theirs won't be.
[1]: https://en.wikipedia.org/wiki/Validity_(logic)