I think there is definitely an element of cash generation in these lanes. Why pay for using a bus lane at a time when the buses aren't even operating?
Even in London where traffic is busy, like Mile End Rad and Marylebone Road the bus lanes are only in force at peak times. For example in mile end (And many other places it is 07-10:00 and 16:00 19:00, much more sensible. Given the traffic flow those times are understandable.