2.5$ a month for the average user of third party apps isn't really egregious. Especially since you can also use your own API key and have something close to 100rq/sec for free.
Obviously for FOSS apps they can't stop someone from recompiling the app with their own API key baked in, but the barrier to entry there is significantly higher.
Now it begs the question: why not force 3p apps to show ads to non-premium users? I imagine control is the answer there. Easier to show to adverts how good one app is doing then verifying proper ad placement in every 3p app.
Given how antagonistic reddit has been to 3rd party apps during this one can assume they would make this explicit if anyone tried.