That's strange, but correct. Populate reads:
"Gain one card from each Action Supply pile."
So what is an "Action Supply pile"? It's one where the randomizer card includes "Action" as a type (+ Ruins, which don't have a randomizer card). That's why the Gladiator/Fortune pile is an "Action Supply pile", and that doesn't change when the Gladiators are gone.
Same is true for other split piles, you get a Plunder from Encampment/Plunder or a Territory from the Clashes, if they are on top when Populate is bought.
"Gain one card from each Action Supply pile."
So what is an "Action Supply pile"? It's one where the randomizer card includes "Action" as a type (+ Ruins, which don't have a randomizer card). That's why the Gladiator/Fortune pile is an "Action Supply pile", and that doesn't change when the Gladiators are gone.
Same is true for other split piles, you get a Plunder from Encampment/Plunder or a Territory from the Clashes, if they are on top when Populate is bought.