Yup, the whole point of the SSH over WiFi stuff in the USBNet hack was to avoid having your Kindle open to the four winds with a passwordless root loging, hence it's only doing
iptables -A INPUT -i wlan0 -p tcp --dport ssh -j ACCEPT
when you enable the K3_WIFI option (while also making the SSH daemon require a valid password).
Feel free to edit this to suit your needs

(L170-173 & L105-108 of usbnet/bin/usbnetwork).