I'm not a fan of the 'closed door' approach to classes, and I've never heard an explanation for doing it that I was willing to acept as viable. The best of the bunch goes something like:
"We're going to invest a lot of time in training you, and in return we want committment. We also don't want you going off after a few lessons and teaching our stuff elsewhere." but even that one isn't very good. I mean, if a class is so much better than everything else people won't go elsewhere because everything else will be inferior.
The usual closed-door thing is more like 'we're rubbish and we can't afford anyone bursting the bubble by discovering competence elsewhere'.
I was once in a position of having to choose between a class that was genuinely very good but didn't allow training elsewhere and everything else in the world. I chose the latter, on the grounds that, well, if I train with you 4 hours a week, I'm my own man the other 164 and in any case... if you go into a room and shut the door, you can't see what's happening outside. And you know what? All that stuff going on outside might be pretty good...