also it's just strange to me that there's so much emphasis on this for ActivityPub / Fediverse but not email.

How many email providers are doing CSAM?

Apple, Google, and Microsoft?

Does Fastmail do it? Protonmail? I suspect not.

What's to prevent a spammer from sending child porn to everyone who isn't on these services?

And how about mailing lists? How many of those are scanned with for CSAM content?