Access to weapons, chemicals, critical infrastructure etc. is restricted everywhere. Even if the degree of access restriction varies.
Why not? Restricting access to information is of course harder but that's no argument for it not doing anything. Governments restrict access to "state secrets" all the time. Depending on the topic, it's hard but may still be effective and worth it.
For example, you seem to agree that restricting access to weapons makes sense. What to do about 3D-printed guns? Do you give up? Restrict access to 3D printers? Not try to restrict access to designs of 3D printed guns because "restricting it won't work anyway"?
The Venn diagram between "people competent enough to manufacture dangerous things" and "people who want to hurt innocent people" is essentially zero. That's the primary reason why society does not degrade into a Mad Max world. AI won't change this meaningfully.
And, yes, in principle, 3D printing doesn't really bring anything new to the table since you could always machine a gun, and the tools to do so are all available. The difference is ease of use - 3D printing lowered the bar for "people competent enough to manufacture dangerous things" enough that your latter argument no longer applies.
FWIW I don't know the answer to OP's question even so. I don't think we should be banning 3D printed gun designs, or, for that matter, that even if we did, such a ban would be meaningfully enforceable. I don't think 3D printers should be banned, either. This feels like one of those cases where you have to accept that new technology has some unfortunate side effects.
The point is that making dangerous things is becoming a lot easier over time.
How can it “inform” you of something subjective? Ethics are something the user needs to supply. (The model could, conceptually, be trained to supply additional contextual information that may be relevant to ethical evaluation based on a pre-trained ethical framework and/or the ethical framework evidenced by the user through interactions with the model, I suppose, but either of those are likely to be far more error prone in the best case than actually providing the directly-requested information.)
Well, that's the actual issue, isn't it? If we can't get a model to refuse to give dangerous information, how are we going to get it to refuse to give dangerous information without a warning label?