I think it is ethical and reasonable to include clauses in the A/GPL restricting where you can host the source code if the hosting service has opensource hostile terms in their ToS. US case law already allows this by treating such license as a "new" license different to the GPL.
(And I don't see why we cannot add a clause saying that the source code is only meant for human developers and use of the source code in any machine learning system or to train any AI systems is prohibited without explicit case-by-case permission).
Has FSF actually said this? I don't see how it makes it a non-free license - the original intent behind the xGPL is to ensure that a user of the software also has the access to the source code, along with the knowledge of who created it (attribution). This is to protect our right to repair. So if any part of your source code is used by someone else, even by an AI system, to generate a software, it should also inherit the same viral property of the license - be licensed under GPL with attribution. If it does not, it is similar to a non-free software using your GPL code in the project, which is already prohibited by the license.
(And I don't see why we cannot add a clause saying that the source code is only meant for human developers and use of the source code in any machine learning system or to train any AI systems is prohibited without explicit case-by-case permission).