No, I don't think it's fair to charge for missed sessions unless the person has a habit of cancelling, leaving the therapist with time he/she can't schedule.
I think it's really not realistic to expect every client/patient to never have a situation where they have to cancel less than 24 hours from their appointment. It's like being punished for being human - and in therapy, of all place!

My therapist doesn't charge, although it is her policy, if she chooses to apply it.
In 5 years, I have had to cancel a time or two... and so has she. We are both human