Certainly, what they called "WiFi Sync" is a misnomer.
It just requires that both the host and device syncing be on the same LAN. At first I thought your request was that they start calling it what it literally is, "LAN Sync"
so I really hope they will change that name.
There actually is a reason why it might work only on someone's WiFi however. The same LAN means both have the same subnet mask and net address. Sometimes the WiFi is a different subnet just to protect local computers from invasion through WiFi.
In that case you could not sync from your phone on wifi to your desktop plugged in. Both would have to be either on the WiFi or plugged in.