Why should the apps have to pay? Why can't reddit provide a paid version that allows you to access the APIs, which the external apps can then use? You already have to log in to them anyhow so it's not like you'd need an extra API key or anything. $20/year is probably an order of magnitude more then they make on ads served to me. The weirdest thing about the ad economy to me isn't that it exists, but as I've banged on…
They already have monetization in the form of Reddit Gold and awards. You can pay for premium right now. Also, people are _really_ reluctant to pay for services they get for free; take a look at the recent thread about YouTube ads [0] and the lengths some people go to in order to justify watching the content for free (while blocking ads, no less).
I don't necessarily disagree with your take, but I'm pretty sure you're overestimating how much people are willing to pay Reddit directly.