The difference is, when you've been a faithful Catholic for 20 years, and tithed the whole time and whatever else, they don't take you aside one day and say "Hey, here's the super-duper secret Bible that almost no one gets to look at. You're going to love the chapter where after Jesus' resurrection, grey aliens from Proxima 9 took him on a 2-millienium mission to the stars."
Scientology does just that. If you have no idea, going in, about what thetans are, or where they come from, you don't find out about them until you're so invested in Scientology that it's very difficult to break away from it. "It has to be true, look how much time and money I've invested in it."
And that's another thing.
Let's say, for whatever reason, that I want to study up on Christianity. Well, one option that a lot of churches have are discussion groups/classes on it, especially for people who are converting to that church.
A lot of those classes are pretty cheap, if not outright free, and here's the important bit. You don't actually have to take them. I could, right now, walk into practically any church in the country and join, for free.
In Scientology, if you want to learn more (or are peer-pressured to do so), every class costs money. The higher you go, the pricier the classes. Oh, but you can get around some of the costs by signing a billion-year contract.
Yeah, that's all completely normal and above board.