"I would think banning something that may be illegal plants you pretty firmly on that slippery slope … "
Not sure I agree there. A forum (non-tech, it's actually a model-specific motorcycle forum) that I'm involved in has explicit rules against discussion of "politics, firearms, and police", not because it's illegal, but because there's such a strong track record of well intentioned discussions of those topics degenerating into arguments and fights that are _clearly_ not worth whatever "benefit" the free and open discussion of those topics on a motorcycle forum might have.
Whether or not you agree that grey-area images of children are technically legal, the owners/admins of Reddit clearly have the right to say "legal or not, we choose not to host those images/discussions here." You wouldn't expect a church or McDonalds to feel happy with you arranging a group of friends to meet there and swap bikini model photos - even if they're completely legal images, they're inappropriate for someone else's venue and they've got every right to say "not here, please". (and have you moved along if you persist).
I think fears of "slippery slopes" here are unfounded.