After reading some comments, I feel like some people kinda forgot the nature of these demons, they are very selfish and greedy, sometimes even uncaring, for example in walter park, when kids are trapped in rubbles, the demons won't help them because there is no benefit in doing so
For the many eared race whose only strength is their hearing, the only thing that they could do is to be a servant for high ranking demon. Unfortunately for them, because of the nature of the world, if their hearing falls short or their hearing is only slightly better than other demons or even decide that they don't want to be a servant, they could be destroyed since they have no use. Due to this, the kids and next generation has no choice but to train their hearing.
Now, this is where Nova and the many eared carrot comes in, since growing the carrots require the use of magic, this could train them to be more proficient with magic and gives them extra income. Instead of selling service, they can sell produce, this in turn allows them to have more time doing something else, since unlike talents, produce can be stored and managed properly, so as long as they are able to follow the demands, they are free to do something else. And with the approval of a high ranking demon, this could be an exclusive item that can be sold for a lot of money and can be sell with limited quantity, and since this is can only be cultivated by a many eared race, they run no risk of competition. with money and time, the many eared race no longer need to be force with only one path, this allows them to choose what they want to do, if they decide to continue their hearing training, they can do that, but if they want something else, they no longer run the risks of being destroyed.
Also, since this is approved by Amurylis, I highly doubt that the asmodeus clan will allow anything bad to happen to them. I think that rather than a servant-master relationship this could be a business partner relationship, where the many eared race provide better deal for the asmodesus clan and the asmodeus clan protect the many eared race.