I can't speak for Iran, but it may be a warning against attempting to land troops on Kharg Island. They're showing that they've been "nice" so far, but they have escalation paths America may not have considered. I think most people thought they were limited to short range missile strikes.

Americans can be convinced of anything without too much effort so that isn’t really a factor here.

Or the US could just stop bombing Iran? Then there would be no reason for Iran to attack American cities.