It's not clear to me why a ticket to London Underground Zones 1-2 would be valid on the Overground to/from London Fields (LOF), but perhaps I'm missing something obvious. There is NR/LU interavailability between Tottenham Hale and Zone 1, however I would have thought that would require a ticket to/from Zones 1-3.
Either way, there is an obligation in the Ticketing and Settlement Agreement for the "lead operator" (i.e. main fare-setting operator) for every journey to offer certain through fares, so the "unfair" through fares cannot be withdrawn - in theory. Of course, in practice we know that this sometimes happens anyway (whether with permission or otherwise) when it's expedient for political or policy reasons, but that will be the reason GWR will hide behind for now. I imagine it will be the same under GBR unless there is wholescale reform of the TSA, which I can't see happening anytime soon.
And although you may consider the through fares to LOF as "unfair", bear in mind that in many circumstances they are actually cheaper. The Anytime Day Return is £80.20 vs £87.90 to Zone U12, and there isn't an Off-Peak Day Return to Zone U12 whereas there is to LOF.
In terms of finding the fare you need to search for "Zone U12*" or similar, see
here.