NSFW subreddits are a black hole in terms of advertising potential. No mainstream company wants their products on display next to porn.
A big subreddit like /r/pics going NSFW, with its absolutely massive userbase, takes out a large chunk of Reddit’s potential advertising appeal.
Obligatory IANAL, but…
Generally a search warrant needs to be issued by a local authority, and that requires the crime to be prosecutable in the place where it was issued.
So in theory, California is potentially able to refuse requests to search for things that are not illegal in California but may be illegal somewhere else.
That being said, it looks like there are specific practices in place making it easier to issue warrants for electronic data like this scenario, even across state lines.
And in this particular circumstance, the alleged offense is even illegal in California (abortion of a viable fetus), so it’s a bit of a moot point anyways. A Californian judge would have issued this warrant if a local police department requested one.