Not true... both Virgin Mobile and Cricket Wireless in the US sell the iPhone 4S as a prepaid phone.
It seems that you are right. However, if we keep on point, we see that prepaid phones do not offer a cheaper alternative. Virgin has the 4s for $650.00. If you trace back the conversation, you'll see that prepaid phones were brought up as a side note. The real issue was whether or not iPhones are as widely available around the world in countries and on network for free with contracts as are Samsung phones. They are not.