Enable internet sharing via iPhone USB
The question is old but it seems it is now possible to share a Mac Internet connection from the ethernet port, with an iPhone connected to the Mac via a USB ↔ lightning cable, 


Ensure the Mac Internet is working properly via the (thunderbolt adapter) ethernet port 
Connect the iPhone (iPad) via the USB ↔ lightning cable
Unlock the iPhone ; if its asks for "Trust this computer?", tap "Trust"
Go to the Mac System Preferences Sharing [ uncheck "Internet sharing" if it is checked in order to allow changes to "To computers using:" ]
Choose "Share your connection from:" either "Ethernet" or "Thunderbolt ethernet"
Check the "To computers using:" "iPhone USB" box then check the "Internet sharing" service.


The iPhone should now be able to use Internet from the Mac.

If you want to be sure the iPhone is really using the Mac connection, switch off "Wifi" and "Cellular data".

[ achieved on iOS 10.3.2, MacOS Sierra 10.12.5 ]