First, to find nudity, you have to go out of your way on 500px. Is it there? Sure. But is a 4 year-old going to run across it without concerted effort? No. And if they do find it, will it really have a negative impact on their development? Highly unlikely.
Meanwhile, you can buy an app that will let you watch UFC fights. It is rated 9+. In case you're wondering whether UFC would be appropriate for 9 year-olds, I refer you to a Google Image Search for the St. Pierre vs. Condit fight from a month or two ago (warning: graphic and bloody) http://bit.ly/VSwUAJ
I agree about the values of many Americans. I am not one of the people who who thinks boobs are particularly harmful.
That said, MANY PEOPLE who have dollars apple wants have those values. So they make a system so their devices get turned into something no more "dangerous" than a Wii/DS, and children get iPads and iPodTouches without parents worrying about corrupting their kids with nudity (again, their values, not mine)