Source, please.
So what if they consult a timetable? 15 minutes is not a long time to wait for a bus, especially in a provincial town. You seriously need to adjust your expectations.
This is one of many previous studies made on this subject.
Most models assume that 5-12 minutes is the transition between random and timetable-based arrival, but they found out that even at 5-minute interval, the arrival to the station still isn't totally random.
The following is my own opinion:
15 minutes is a long wait for a local route (for example, an estate-to-market route which is only 2-3 km long), which is unattractive for short distance passengers.
In mainland China, people complain if their local route don't come in about 10 minutes.
In Hong Kong, reducing frequencies beyond 15 minutes in the urban area is deemed to be an action which the bus company is giving up on the route as people will less likely to wait for such routes and use alternatives instead.
In London, there are very few routes which run at less than every 15-minute daytime frequency.
All the above evidences, and the various studies quoted above, indicate that 15 minutes headway is not a truly turn up and go operation. If transfers are needed between two different lines both above 15 minutes, but different headway, it will be very unattractive. Examples can be found in Hong Kong where some interchanges have little use by passengers because all the frequencies of passing routes are 15 or 20 minutes, making changing buses unattractive even if changing means more direct, while at other interchanges where there are multiple routes combined to serve a place making a turn up and go frequency, a lot of people will rather get on the first bus to the interchange and transfer there if the direct route isn't a turn up and go frequency.