Most of the people that buy Tacomas and even Tundras never tow anything, use them to commute to work.
And chances are anyone that tows seriously will add an auxiliary cooler on their dime.
So probably a accountant determined that saving money on a air cooler for all their cars is more money than the few warranty calls for burned transmissions.
Now, the heat exchanger is still technically a "cooler". It just cools the transmission with a fluid at 185-190F. So obviously, that's the lowest the input fluid can go, so the transmission will have internal temperatures higher than that.
But 260F (like the OP corrected) is killing the seals, clutches inside transmission. As for the fluid itself, a synthetic fluid might be OK, but Toyota WS is not that.