Why is a non-cellular iPad model announced as new network interface when connecting to a Mac through USB?
You are probably referring to this message:


From today's point of view, your Mac's behavior is a bug. Maybe the iPad advertises its Bluetooth to be a network adapter? Like WiFi or cable network adapters, Bluetooth chips have a MAC address. Here, MAC doesn't refer to Apple's computer brand, but to a universally unique identifier. See for example the https://macaddresschanger.com/bluetooth-mac-lookup

iOS does support the Bluetooth personal area network to route data packages to other devices, potentially connecting to their internet:
https://developer.apple.com/documentation/multipeerconnectivity

Theoretically, the iPad could connect to the Internet via a Bluetooth-based personal area network. Currently however, the iPad's Personal Hotspot feature until iOS 12 only supports sharing cellular data. I am not aware of any plans that might change that. 

Maybe your Mac is ready for the day a future iPadOS or iOS version extends the Hotspot feature to Bluetooth?

You can try to log into macOS using an admin account. Maybe the Network Preferences sidebar allows you to delete the iPad. If yes, then you shouldn't see the message anymore when connecting it via USB.