Copyright laws don't cease to be a thing the moment you post something on the internet. The words are still yours. If this was X/Reddit/Facebook or the like instead of Bluesky the researcher would have immediately found himself on the wrong end of a DMCA takedown request and maybe even a lawsuit.
A lawsuit such as LinkedIn v. hiQ ?
The concluding scraping publically accessible data was not a violation of CFAA, after which Twitter et al went logged-in-users-only?
I don't know if copyright comes into this. While social media terms of service are very clear about licensing the comments of individual users, that's to protect them from what the law doesn't say implicitly. Is every comment posted to Twitter or Bluesky a "literary work" ? An original, creative expression ? I have my doubts but I guess there's room for a lawsuit yet.
Why are Google, OpenAI etc. buying user generated data from Reddit and other similar sites for hundreds of millions of dollars a year? If Reddit comments aren't copyrighted and aren't original creative work then anyone should just be free to scrape them directly right?