Who ever came up with that price is looking for short-term profits over user happiness and long-term growth.
So, if you already have a sophisticated ad tech and sales team, you'd be able to pull 50%+ profit margin without having to worry about running the infrastructure for content.
That being said, there's maybe only a handful of companies with a more competent ad tech/sales team than Reddit, and Reddit's is pretty damn bad. So while the numbers make sense, the strategy does not given the competencies available in the market they're trying to sell in.
they're passion hobby projects that'll disappear rather than turn into a job
If you're building your business to be completely reliant on another unsustainable, unprofitable business, don't be too surprised when they ask you to help row or get off the boat before it sinks.
For API restrictions, Reddit has been in a doomed if they do, doomed if they don't situation for a while now. I think there's about a thousand other better decisions they could've made before being forced to make this one about API usage, but I also don't see their numbers and their time simply might've already run out.