I personally disagree, but the answer to this question is the crux of the case. Visa is allegedly a co-conspirator because by choosing to providing a platform, Visa also benefits from the traffic generated by CSAM on Pornhub. Visa doesn't directly encourage or control what is uploaded to Pornhub, and no one is suggesting it does in a legal context either. But, Visa knowingly choose to continue working with Mindgeek properties after a NYT op-ed came out highlighting the CSAM available on the platform. There's no "alleged" with respect to Pornhub, it did host CSAM and it did make money from advertisements, and according to the case filing, also had internal software to tag and promote CSAM algorithmically. Visa profited by providing a merchant platform to the advertising arm of Mindgeek despite knowing CSAM existed.
I don't agree with the overall precedent that would be set, but it's hard to argue when Visa chose to stop working with certain parts of Mindgeek showing they knew CSAM existed. Visa made itself a moderator of content by deciding who it does and doesn't work with based on the kind of content they serve and that decision exposed them up to the consequences of Mindgeek's actions.
Payment processors are in a difficult position but I believe they should be treated like a utility, similar to electricity. If net neutrality was solidified in the US, Visa could potentially argue that it treats all data in a similar way and then the lack of moderation and Pornhub/Mindgeek alone would be at fault. But by by choosing what should and shouldn't be allowed on the platform of its customers, Visa becomes a responsible for when its customers break the law using its platform.