One party is blatantly opposed to courts and legality and law. The idea that the ABA should be obligated to be non-partisan against their interests makes no sense. Nobody "owes" both sides anything. This logic would also not make sense if leveled at the NRA, farm groups, etc - or any other group with a partisan lean.
If one side is just plain wrong - they are not owed 50/50 representation. And accusing them of "reverse discrimination" is ironic if you are also going to criticize them for not forcing inclusion of a political party.
To put it another way: if the American Marxist Party was to suddenly become an important party, would the ABA gain authority by catering to them? In the name of non-partisanship? It makes no sense to me.