They don't need to be. Showing ads to users isn't Reddit's only revenue stream, and they're only requiring it out of greed. The whole point of API is to make it easier and cheaper for things to access reddit's database, without API then other services will scrape from the website, which uses far more of reddit's resources. And that's before you consider the fact that reddit's API charges bear no reasonable proportionality to any actual costs.

Blocking third party apps or requiring apps pay for API access and pass the cost on to the user is akin to blocking or charging users to access the website while using an ad blocker. Yet, users are apparently far more forgiving than they are with sites that block ad blockers.