That's not direct comparison. The equivalent is the ORIGINAL text being published and purchased and then a translation acquired- not bought. There is no 2nd book that is being sold- it's more like leaving scribbles in the margins. They are essentially being sued for a mod. Maybe they are within their rights to do so; don't care, it's bullshit.

Let me ask you a question: can you publish a mod where the copyright-protected script is changed to say something else entirely? Not a translation, just something else. How about the exact same, but a different colour? You have a better argument with books since it's difficult to distribute a translation without printing a copy that by nature doesn't need the original to exist. At best you are arguing that the law is inappropriately being applied for emergent mediums/cases that the original legislation doesn't properly address.