You get stuff like... obnoxious notifications?

Duolingo's notifications are borderline emotional blackmail ("don't make the owl sad!"), and Duolingo is a vastly profitable company that expressly targets school-aged children. But because it's not social media, it's... fine?

What does playing fair even mean in this context?

It's not just that, it's also outright black hat behavior like https://news.ycombinator.com/item?id=4151433