The users don’t belong to Apollo. Reddit provides a service to Reddit users, Apollo freeloads on that service, cuts off Reddit’s revenue stream, and replaces it with its own. Why would anybody think that Reddit has some obligation to allow this, or that Apollo has some right to do it?
Nobody thinks that, not even the Apollo author, which is why the problem is not that Reddit is charging for access to the APIs, but that they are charging far more than the amount they’re losing.