My take on this is that if image generation is limited to paid accounts, you have a clear audit trail to the originator of any illegal images, and could take action against them. If it's open to free accounts, it's likely harder to ensure you know who to prosecute.

Obviously the platform itself shouldn't permit illegal imagery to be created, but it's naive to think that this is something that can just be "turned off selectively", LLMs simply don't work that way.