tip: lex m/m (mature OR explicit)
Because the law is new and there's been no enforcement yet, no case law, no precedent. How it will be interpreted is still a bit of a mystery. The sites you're referring to may well be violating the law, depending on how it's interpreted in the future, and that may mean the end of that site. (Are they registered non-profit organizations with legal counsel?) It seems like AO3 is taking a very cautious stance here, and it is indeed frustrating, but they allow an extraordinary amount of "anonymous" interaction in the form of guest kudos and comments, so it appears they're taking a wait-and-see approach rather than hoping things will work out in the end.