Their argument is that since it's centralized, things like that are possible (while with llama2 you can't), they do "patch" things all the time. But since blobspam are contributing to paying back the billions microsoft expects they're not going to.
Unfortunately, any question that lots of people legitimately ask will also be a prompt for blogspam.
Also, it would require OpenAI to store the history of everything that it's API has produced. That would be in contrast with their privacy policy and privacy protections.
If it's a perceptual hash, that's easy to _exploit_: just ask the AI to repeat something back to you, and it typically does so with few errors. Now you can mark anything you like as "AI-generated". (Compare to Schneier's take on SmartWater at https://www.schneier.com/blog/archives/2008/03/the_security_...).