Why is it unsupported? Pretty much all x86 base drivers work in Windows RT.. except Ethernet.
It's surely not an ARM restriction - there are ARM devices with Ethernet.
Android can use Ethernet adapters, and that thing is a pure mobile OS! Yet Windows RT, a subset of the full powered Windows, can't! You can't tell me it's because of technical issues (the fact that third party drivers work [which get taken down by MS?!] is proof that it's no technical issue).
Microsoft has deliberately crippled this feature on Windows RT on purpose. Reason? Forget my "conspiracy theories" if you want, but fact is that Ethernet is crippled on purpose, and Microsoft is putting effort to let it stay crippled. Why the heck is this the case? I am not saying that my explanation for Microsoft's motivation for this is perfect, but the issue exists nevertheless.
And again I have to quote myself:
Sure, you can turn off the Wi-Fi router, but then you disable all connections, for all people in the house (often these routers are a combined device - you turn off your TV and phone connections too)
It's more of a hassle for sure turning off the router.
This affair is not a perfect plan by any means, just yet another stepping stone for forced cloudification. A piece of the puzzle, not the full thing.