> It will be difficult to claim fair use if training AI model is explicitly mentioned in the license, I think
Fair use is a statutory right (codifying what courts had 0reviously found to be an aspect of Constitutional free expression rights limiting the copyright power) that limits the exclusive rights of copyright owners. It can't be reduced in scope by license terms, because it deals with what the owner has no right to control in the first place.
(You may be confusing “fair use” with “implied license”, and, yes, explicit license terms would be a powerful argument against an implied license argument.)
My impression, they claim the fair use only because there is no other ground for ML training on GPL code. The license only describes other uses of the code, so the first question is on what grounds Open AI uses the code at all. And the only thing OpenAI comes with is the fair use.
ML training does not fall under free expression or similar basic rights, I think. Only because it looks too restrictive to forbid scanning the code which is publicly available for people to read, and the license itself does not mention such use, the fair use may be considered as some kind of justification.
I, of course, can be mistaken. Especially that I didn't study this subject deeply.
Also, for authors who are against the ML training on their GPL code, it's better to prove it violates even the current GPL version, rather than just introduce a modification to the license. Because new license will only cover new code, while old versions are already available under the old license.
On the other hand, such an amendment to the license will not be harmful, even if later proven to be useless. And it will clearly state the copyright holder's position regarding the ML training, instead of some subtle reasoning we should apply currently.
Some people may want an opposite amendment to their license - ML training does not produce a "work based on the Program" and can be freely performed by anyone.
Fair use is a statutory right (codifying what courts had 0reviously found to be an aspect of Constitutional free expression rights limiting the copyright power) that limits the exclusive rights of copyright owners. It can't be reduced in scope by license terms, because it deals with what the owner has no right to control in the first place.
(You may be confusing “fair use” with “implied license”, and, yes, explicit license terms would be a powerful argument against an implied license argument.)