According to Apollo's developer 80% of the users make less than 500 requests per day, so I'm guessing the proportion of power users making a lot of requests are in the single digits. I doubt enough of them want's to pay enough to subsidize the others.
There's also the point that using Reddit is a two way street. Reddit is made up of user created (or user stolen) content, and moderators are working for free. Reddit's whole value is made up of user interactions, allowing users to make those interactions is not just a cost.
Why should the app developer pay anything at all? The users are also Reddit's users, they authenticate and can use Reddit's resources in a number of ways, why not charge them directly? Will Reddit be sending a bill to Mozilla for my page loads?
Because Reddit makes money off the ads. They can't guarantee third parties will always show the ads in the way they've designed.
So now they are charging third parties the cost of losing ad views.
Not all apps are worth $5/month, but this particular one that people apparently love enough to spend hours per day on certainly seems like it should be. A slightly nicer calculator app that you use twice a month? $0.99 is fine. A professional productivity app that saves a high-value worker hours of time? Way more than $5/month.
It's ok if not everyone can afford an app.
> Why should the app developer pay anything at all?
The app developer is just a proxy for the users, who are using Reddit without seeing any of Reddit's ads.
And then Reddit would have to audit third party apps to ensure paying users are getting what they pay for. Sounds more onerous than just making third parties pay up.