Probably a U.S. cultural thing. I find it weird talking to other parents' kids too (I'm a father of two), as you don't know how the other parents will handle it. I'd like to think there are places in this world that aren't so paranoid.
It's not just a U.S. thing. I'm a UK father of 3 small boys, and it's a given over here that you should be very wary of interation with any child unless you have some kind of relationship with their parent. It's somewhat relaxed where parents feel more comfortable; a zoo might qualify for that.