If, as it would appear, the data is polled – all stations every ~3 minutes, then on the face of it as you say this is very inneficient.
A lot of docks may have no change in this period, while others may have changed a great deal. I have worked in the past with traffic survey units that use a GPRS modem, keep a constant TCP conenction open, and send data as an event occurs (car passes a loop) – similarish situation to the cycle hire requirements.
With this model, you get updates as and when they happen, and when not much is happening, there is no data cost incurred. Seems a much better approach – but there may be some reason TfL haven’t gone down this route – be it technical or contractural…