[h2odragon]

It could be worthy just as a contrast to all the other apps using addictive patterns for other less beneficial uses.

The ethics of it are neutral if the user has the controls. If someone else has the controls then there still might be ethical uses. Which particular instance of usage vs. bait and reinforcement would depend on the context of the ongoing process between the user and the provider...

Much like we require some medical therapies to be monitored and prescribed by trained and licensed providers; perhaps the dangers of unethical use here would require society to be involved in the form of regulations and professional standards.

"app" is a distraction; irrelevant to the question of ethics. Can you as an individual go teach someone a new thing that brings them joy, praise them and otherwise reward them in ways they find pleasant, without expecting to face social consequences? If it's wrong for an "app" to do it it otter be wrong for you, as well.

Would it be right to just go teach someone else's kid to play drums, without any oversight or preparation for the negative consequences? Dow we really want our society to encourage the undisciplined spread of libertine free-thinking like that?

ask "what is truth?" and see also: https://en.wikipedia.org/wiki/Order_of_Assassins