Thank you for bringing this question. From the other side of "the pond", I have noticed the same trend as you have. Due to the important administrative costs of this (management of registrations, waiting lists, cancellations, etc.) and knowing that the venue requires us not to exceed a certain number of people, we have, for this Fall, asked for a small fee that is then deduced to the class fee if people choose to register. And to my surprise, we still have had quite a number of last minute cancellations and no shows.
I feel there is no "perfect" procedure, and also I'm curious about this trend that I've also noticed. I also wonder how it works for other MBSR teachers.