It's not the specific thing being made illegal, it's the underlying philosophy of "Better a dozen innocent men go to prison than one guilty man go free" I'm arguing against here. Most western justice systems operate under a principle of requiring guilt to be proven beyond a reasonable doubt, and if there is doubt then guilt cannot be considered proven and the person is not convicted.
The comment I'm responding to is proposing a situation where non-AI-generated images are illegal but AI-generated ones aren't, and that there's no way to tell the difference just by looking at the image itself. In that situation you couldn't convict someone merely based on the existence of the image because it could have been AI-generated. That's fundamental to the "innocent until proven guilty beyond all reasonable doubt" philosophy I'm talking about, to do otherwise would mean that innocent people could very easily be convicted of crimes they didn't do.