I suspect it's more a business restriction than a moral one. If OpenAI allows people to make porn with these tools, people will make a ton of it. OpenAI will become known as "the company that makes the porn-generating AIs," not "the company that keeps pushing the boundaries of AI." Being known as the porn-ai company is bad for business, so they restrict it.

The part I really don't understand the purpose of the threat "Further policy violations may lead to an automatic suspension of your account".

Why do that? Just refusing to run my query is sufficient. Who is harmed if I continue to bang my head against that wall?

Because it is a solid line (the policy rule) drawn across a fractal boundary (actual porn), and given lots of attempts you can find somewhere inside the line but across the boundary.

Stopping more than so many attempts makes this much harder / much less likely.