I have never been to a pt, but I have worked in an eye dr office and I know from experience that doctors overbook for a reason. So many times people just don't show up for appointments so doctors overbook to make sure they have a full schedule. The problem happens when everyone actually shows up.
As for thinking they can bill your insurance for a missed appointment, that is just not true. It's hard enough to get insurance companies to pay for legitimate expenses, they for sure will not pay for appointments that never happened. Doctors are allowed to bill the patient for missed appointments, but usually what happens when they try is that a patient does not pay and decide to take their business elsewhere. I realize that the doctor I worked for was in a large metropolitan area and patients had many other options to choose from so that may not be the case in more rural areas.
I agree that it is frustrating for patients when the doctor runs behind schedule, but it's also frustrating for the staff who have to deal with it all the time, so try not to take out your frustrations on them.