That said, if you click the link next to BadUSB, they detail attacks whereby the device pretends to be a USB Ethernet adapter instead. And while you're right that stuff typically wants user input prior to connecting to WiFi networks, I don't think anything prompts before connecting to wired networks. The onboard WiFi could even make it appear to work, so as to not arouse suspicion (by simply bridging the pretend-ethernet to the WiFi), but now your attack has a MitM and a keyboard…
Needless to say, you don't want random USB devices getting plugged into your machine.