If that's the reason why this specific driver was pulled, ok.
But it shows that there is no technical reason to exclude Ethernet drivers. The fact that the driver does work (albeit with some a few bugs maybe) is proof that there is nothing in Windows RT or the ARM architecture or whatever to prevent Ethernet.
So Microsoft not making the Ethernet driver from W8 available to RT is yet another arbitrarily restriction for no good reason (if MS doesn't want to "waste" the few MB, they could put it on the download center).
There's no Ethernet port on the Surface RT, so there's no driver...
But I think you meant to say "Compared to Windows 8 (e.g. Surface Pro), I don't understand why Windows RT (e.g. Surface RT) has a higher bar for device driver certification".
I suspect it boils down to: battery life (or The Experience). Windows RT was created for mobility scenarios where users want/expect battery life parity with, say, a smartphone. Or iPad. A device driver that doesn't follow the rules drains battery and impacts those very scenarios. The Experience is then degraded. (This is also why we don't get desktop apps on Windows RT -- too many power hungry unoptimized APIs, not enough testing by the devs, etc...)