"Allowing to use an ethernet adapter" translates to me as "allowing to use the internet" on the device.
I would think that's part of this. One does not equal the other. You are admitting the adapter works. So you have not been wronged.
Let me leverage my go to analogy for this...
Just because your car hauls 4 people comfortably doesn't make it a truck for hauling lumber.
iOS was never really designed to use Ethernet. I'll wager a guess there is no "make sure it still works over ethernet" team in the iOS camp. That you have found a kludge which allows it does not, IMO, compel Apple, or any developer for that matter, to provide every last bit of compatibility some users desire.
The need for Ethernet is not lost on me. Some users for various reasons do not have or believe in wifi. So be it.
That belief and choice has its costs.
No FaceTime over Ethernet seems to be one of them