The availability of the copyrighted works is not determinative. Fair use in the US takes (at minimum) four factors into account, listed in the federal copyright statute: https://www.law.cornell.edu/uscode/text/17/107.
That quote from Stanford's library is not discussing fair use doctrine in general, but rather is stating what is permitted in those specific circumstances. There are plenty of instances of fair use where the underlying work used was available at a fair price. That's the whole point of fair use law: some use of a work that is facially infringement escapes liability because the particular use is considered fair.
Current US copyright law is not clearly in a place to view model training as infringement. Courts have a long history of permissiveness in the face of copyright challenges to new tech (e.g. the image search engine cases, Google v. Oracle and smartphones, Sony v. Universal and VCRs) and I predict it will happen again with AI. The cat is out of the bag and judges know that finding training to be infringement of each training example will have a negative impact on a new product category. If training was more obviously infringement then that permissiveness would be harder to sell, but in my opinion it's really difficult to argue that a "copy" of an example has been made during training (aside from the copy made to process the example).
Another aspect of this arrangement: you don't pay federal income tax on money you receive as a loan in the US. The money does not count as income because of the matching obligation to pay it back.
Nothing you mentioned has anything to do with communism AS AN IDEOLOGY, though. So why discuss it? It could have been any group based on the logic of your comment.
I'm too lazy to check the GPL comment (I'll assume I made a mistake). But as far as I can tell my only reference to Stack Overflow was not about liability based on copying from SO. I was making a comment about a common industry practice.
I think some countries use a different tradeoff than the US for their income tax. Instead of spending a ton of government and taxpayer time and effort to accurately assess what each taxpayer owes, the government simply generates an estimate based on what it knows about each taxpayer and uses that. They accept the reduced accuracy but it is offset by the reduced effort in determining everyone's bills. That is to say, you don't need to move away from an income tax to address the problem.
Sure, but where the burden lies still matters. The plaintiff needing to prove copying is a lot more defendant-friendly than the defendant needing to disprove copying.
The simple answer is that the Supreme Court of the United States said as much. Lower courts are bound to follow SCOTUS' holdings. Of course, precedent can always be overturned (i.e. if SCOTUS changes its mind), but the N&P Clause's interpretation is unlikely to be changed.
Congress has a lot of room within a grant of power (e.g. "to promote the progress...") because of the necessary and proper clause. In short, the tight relationship you're demanding is not required.
Copyright infringement is always in civil court. What I was getting at is that you can view a license violation as a contract dispute or copyright violation. The latter may mean high statutory damages (i.e. a pre-set range of money values that can go as high as $150K per work infringed), whereas the former does not.
That quote from Stanford's library is not discussing fair use doctrine in general, but rather is stating what is permitted in those specific circumstances. There are plenty of instances of fair use where the underlying work used was available at a fair price. That's the whole point of fair use law: some use of a work that is facially infringement escapes liability because the particular use is considered fair.