Well, it's not like going to your dentist, or hair cutter, or tax preparer and find your appointment was given away. Every Asian Massage Parlor that allows walk-ins has experienced mongers making appointments and they never show up. I bet it happens frequently, and they have no recourse. Guy walks in, with cash in hand, they go with the sure thing. They are not telling him to leave and hope the monger who called hours ago is definitely showing up.
Best practice is to plan no more in advance than the time it takes to arrive at your destination. Literally in your car, engine running, solidify the appointment and say you'll be there in 20, 30, etc. minutes. Even then it's a crap shoot, but you've stacked the odds more to your favor you will get the girl you want.