But in other cases, shouldn't it be both? OpenAI is ultimately the one executing the model calls. It's not like they send you a hard drive or standalone box and then you use it how you want. It's all (intentionally) centralized to them, in a way that's core to their business model.
If they knowingly allowed use of their services for illegal purposes then yes, but in so far that they provide a service that can be used for useful things (including cyber security research) and did a best effort attempt at abuse, I don't see why it should make sense to hold them liable. This is especially the case now that frontier LLMs are almost a commodity that can be used without restrictions from providers outside of your legal jurisdiction.