From T's perspective, that slot has been set aside for the patient, foregoing the chance of getting the fee from another patient. There is a contract between the patient and the client. Though the patient does not come for the session, the T still incurs the cost of the office, insurance,etc. in addition to the T's wasted time. Just because the patient cannot keep up his end of the bargain does not mean the T will not. If the patient cancels before 24 hours, the T has a greater chance of recouping the loss by scheduling another patient in the time slot . Additionally, the T must maintain a certain level of income to continue providing his services to all his patients.

But personally, I'd be annoyed at paying when it comes to sickness, though Ts have no way of knowing if the patient is truly sick, just being resistant, or preferring to go to the beach for the day.