do you mean that even the UDP packets with "wrong" destination address are dropped by the wiznet?
Think lower - layer 1. Packets with the "wrong" MAC address are dropped by any NIC (unless it is in promiscuous mode). A "right" address is the NIC's own MAC, a broadcast destination, or a multicast destination MAC that the NIC is subscribed to (went a bit pedantic on that last one).
-j