When a bus isn't a bus

How come you can have buses with their little sign above the windscreen saying "Not on Service"?
It is a bus. It has a driver. It is going somewhere. Why can't they pick people up?
'Not on Service' buses always seem to be in weird places - a trip on one sounds much more interesting than taking the same old bus route.
These days the signs are electronic presumably they can write whatever they want up there. The old days when they just had a series of signs stuck in a little window and they had to manually change the sign over are long gone. Now they can write, "Off to help a mate" or "Going for a warrant" or probably more helpfully the street where they are going. I'm sure there might be someone who will be happy to catch a ride - to finally find a bus going where they need to go.
If cars with only one person is bad for the environment - what about a whole bus with just a driver?
So good for the environment, payment for the trip and company for the journey - Buses should always be on service.