“Straight-forward” may be too strong regarding these books. If they inadvertently picked up unauthorized copies while scraping the web, that would definitely not be a problem. That’s what search engines do.
The question is if it is a problem that the researchers knowingly downloaded these copyrighted texts. Owners don’t seem to go after downloaders. IDK if there is case law establishing that the mere act of downloading copyrighted material is infringement. I don’t think there’s anything to suggest that knowing about the copyright status should make a difference in civil law.
In any case, researchers must be able to share copyrighted material, not just for AI training but also any other purpose that needs it. If this is not fair use, then common crawl may not be fair use either. IDK if there is case law regarding the sharing of copyrighted materials as research material, rather than for their content. But I find it hard to see how it could not be fair use, as the alternative would be extremely destructive. So even if the download would normally be infringement, I doubt that it is in this case.
Eventually, we are only talking about a single copy of each book. So, even if researchers were forced to purchase these books, all of AI training would yield only a few extra sales for each title. The benefit to the owners would be very small in relation to the damage to the public.