Compare that to the other religions. To the best of my knowledge, there is no super-secret ultra-eyes-only version of the Bible that only the elite Christians get to read. There is no "not for the viewing of non-believers" version of the Qu'ran that only the most devout Muslims get to read. But there are secret Scientology documents which explain core beliefs of Scientology that the general rank and file of the CoS do not have access to.
Unfortunately, it's a bit more complicated than that. Esotericism is, at least historically, a common religious practice. Gnosticism, Mormonism, at least a few Buddhist sects, and arguably the Masonic tradition all spring to mind. All of these have the idea that there are truths which should not be made available to the uninitiated, as they are not prepared to receive them correctly.
So this is the complicated problem: there are no really good grounds for condemning Scientology as a religion. The problems arise, rather, from the Church of Scientology as an institution. Letting aside the heavy-handed tactics used to recruit new members and to protect the Church, the fees charged for initiation seem to shift the practice from esotericism to exploitation. It's worth pointing out that very few people have objections to the Free Zone, emphasizing that the primary objection to the Church of Scientology is fundamentally organizational, rather than religious per se.