The license goes beyond redistribution. You are granting a sublicensable and transferable right to your content, giving the platform the legal authority to sell or license it (or to not license it) to AI scrapers and other entities. The platform's right to block said scrapers comes from posession rights.
Its like if you made a painting and put it in a museum. You still technically own the copyright, but the museum owns the building. They can lock the door, charge admissino, kick out anyone they want, prevent anyone they want from seeing it, etc. You licensing it to them makes it their private property to do with what they wish.
> I can't tell them "yeah it's fine to scrape my comment, don't put up any captcha walls".
Correct, because you signed away that control.
> what if I dedicate my comments to the public domain?
That means you forfeit copyright, but you cannot waive the platform's rights regarding their servers.
But, because you still retain copyright (or in the case that its public domain), you can and are welcome to submit it to AI companies yourself. Just because Reddit may not allow a scraper, that doesn't remove my right as the copyright holder to re-submit my comment to another platform that does allow the scraper.
The difference with Anthropic/LLM output is that there are zero intellectual property rights over the outputs once they leave the API endpoint.