To me it is all a matter of timing: when the pro let's me know with some notice (at least a few hours) I think it is acceptable. Over the years, I have had three occasions where the person just didn't show - no text, no notice, no nothing.
Recognizing that escorts are human, too, I do think it is reasonable to hold them to a higher standard with regard to cancellations -- after all, they are the professional service providers, we are the customers. Like it or not, when you are in a service business, you've got to be flexible, patient, and forbearing. If you can't be, you are in the wrong business.