Aren't they effectively just offloading this whole question onto the apps? For the sake of argument, let's say what they are charging for the API is about 80% per-user of what they make for users who use the official app (and therefore see ads). I have no idea what the actual numbers are, this is just theoretical.
In that case, app developers have several options:
* start showing users ads, and use that to pay both themselves and Reddit
* start charging a monthly fee for the app, and use that to pay both themselves and Reddit
* some combination of these two (e.g. pay a subscription for ad-free use)
Sure, Reddit could make this easier for app developers, but isn't it all basically the same thing at the end of the day? Reddit wants (or needs? I have no idea what their financials look like) to make a certain amount of money per-user or per page view. Apps take home ~100% of their profits currently, and make Reddit ~nothing. So Reddit is pricing in a profit rate into API access.
I mean, just to look at Apollo, they have 166K ratings on the Apple App Store, and surely far more users than that. Reddit wants $20M a year from them. That's high, maybe too high, but how does it compare to the value of (say) a million users a year on the official Reddit app? If Apollo switched to a subscription model on which they charged $1 a month to users, would they be able to pay Reddit's API fees? (Assuming those API fees would drop by at least 50% after non-paying users quit using Apollo.)