There's only two instances where I can see someone charging without allowing for people to look in. The first is that it's part of some military/militia group that's private and needs either some sort of clearance or vouching program to view/participate. The second is if the person isn't really looking at it as a school, just teaches friends/a specific inner circle, and really isn't looking for new students, but is open to you coming to the group classes if you pay. The first one I personally don't have a need to join but I can see how a group might want their secrecy for whatever reason. The second situation, I can see myself going to if the person comes highly recommended by a martial artist I trust, and it's clear that the reason for it is just that they don't see any reason to expand/promote for new members. Outside of that, if I'm not able to do a trial class, or at least look in, I'm not paying.