Maybe I'm missing something but I don't even see any regular porn which used to exist on my Global. Haven't updated my relays for a while so perhaps that's part of it.

CSAM might just be an angle to attack nostr which due to most people's distaste for it leads people to assume it exists without seeing for themselves. I mean who wants to check that! Seems to me pretty straightforward with today's clients to know which relays are sharing what you don't want to see. I'm pretty sure any of the established relays would suffer were they hit with a sustained attack of such content, so the incentives there are functional.