It's really not.
He used an API Reddit was happy to offer for free. Reddit offered it for free because they benefitted from users engaging with the platform through third-party apps, much like Twitter did back in the day.
Nor was he ever asked to.
Arguably yes, his app made it easier for people to use Reddit, increasingly the likelihood that they use the app and contribute content (you know, the entire value prop for the platform).
Not an argument anyone is making, so I don't know why you think this is a valid argument.
It's not a bug if it's an intentional symbiotic relationship. Apollo benefited from Reddit, Reddit benefitted from Apollo (and all other third-party app devs).
The complaint isn't even that they're charging for the API, this is something most informed users agree should happen. The dev even acknowledges that it's in his best interest to pay for API use. The issue is with the pricing structure. Reddit is asking for more than 20x the amount they can reasonably expect to generate from an API call. Imgur, an example provided by the Apollo dev that sees similar API calls for arguably heavier traffic since it's all image-based charges $166 for what Reddit is proposing charging $12K for. This is basically straight from Musk's failed Twitter playbook. You can see it explained far better than I'm able to
here.
It's blatant greed because they need to juice their numbers before they their IPO.
They also benefit from not paying moderators, etc to manage communities, many of whom rely on third-party tools using the API, and many of these tools will quickly go away.
To be clear, Reddit is a private company and they can charge what they like, but they themselves communicated to developers that their API pricing was going to be reasonable and based in reality. It clearly is not. As a result, end-users are being affected because the tools they use to interact with Reddit are going away, leaving only first-party options that are markedly worse. This is why most people are unhappy. It's a clear lack of care for the users whose content is the entire value of the platform.
There, now you can't feign ignorance and continue your bad-faith arguments. You're welcome.