Everyone's time is valuable. If I were to get up early, refuse social opportunities I would have otherwise enjoyed, and spent several hours preparing to teach a class, I would be very upset if no one showed up because 'something came up.' Often, what comes up is something in the line of, 'I didn't feel like going out today'; 'my daughter asked me to go shopping'; 'a friend called to go to lunch.'
No-shows are a nightmare in many professions and that's why people charge a fee in advance. I'm with those who charge a fee for non-emergency no-shows.