Yeah, it will be disabled again, as in vanilla, in the EMF beta tomorrow or tonight. That said, only allowing players to spend prestige for event troops really does seem ... lame. I could give the AI no advantage in cost and keep it enabled, but I think that players would still be unpleasantly surprised by 2500-stacks popping-up (just fewer), although they should expect as much since they can do the same thing.
EDIT: As to why I gave them reduced prestige cost compared to the player in the first place, my reasoning was that it had been disabled in vanilla for the AI because the AI will always spend all of its prestige (lack of prestige-spending reasoning). Thus, I cut their cost. It only occurs to me now that the AI would presumably just still spend all of its prestige (but get more troops for it), assuming they had a lot of it.