You need an API key to access the reddit API. There's no MITM, it's just that requests made through Apollo are tracked to Apollo. And Apollo will be charged for those requests.
The “free” limits are per-app, not per-user. Any API call using Apollo’s OAuth client_id and client_secret is attributed to Apollo’s limits and then API usage, whether it comes directly from a user’s device or through another server.
Reddit has never provided access to its ads to third-parties, and now third-party apps are banned from showing any advertising at all.