And we don't know who the current rulers of the various nations are and whether they're benders. (The current Fire Lord is Zuko's daughter, who's likely to be a firebender, but that's unconfirmed.) All we know is that the United Republic of Nations, created from the former Fire Nation colonies within the western Earth kingdom, has been ruled by an all-bender council during the timespan of TLOK Book 1, though I'd expect that to change in the near future
Do we know that everyone on the council are benders? Obviously Tarrlok and Tenzin were, but if I remember right, we didn't see any of the others ever bend. Also, Sokka and an unnamed Air Acolyte were on the council 40 years before, and clearly neither of them were benders.
I'm not saying that they weren't
all benders; I just question the assumption that they were.