The reasoning for moving fry is that they are not settled into the tank and not yet fully aware of things but the older pair will be used to things. So a move is not much for fry, while moving a breeding pair will set back a bit their work to get ready again . In nature, it often comes down to what is expedient without any moral objections or thought. So if the female is needing to fatten up again, it requires food and if the most available food is the fry, the timer is running and if they are not out of sight as they would be in nature, they become a way to fatten the female quicker. How long or how soon is not a precise thing but just something that happens at some point. Some fish are more inclined or there are times when something happens in the room which triggers the reaction. With newer, less experienced adults, it seems to happen more often or quicker, which is part of my reasoning for leaving some of the fry with the parents as it seems to firm up some of their actions for the next time. I doubt that the thought is the same but it may works that if all the fry are suddenly gone, why not just eat them immediately the next time?
Not a lot of firm science in all that thought, just what I feel works better! Also have to admit that part of the plan is that I can fit another ten gallon in for fry where it is more difficult to fit another tank for adults while leaving fry in the larger former tank.