That's inbound traffic coming from the outside world. You need NAT because the load balancer only knows about nodes, not individual pods (perhaps you can pull it off with e.g. ELBv2, but definitely not with v1).
There's more iptables magic if you talk to a service's virtual cluster IP, because of the load balancing, but from pod to pod, which is what I thought you were referring to, NAT is usually not involved.
