It seems I was incorrect in thinking a Flying Motorbike sign could not be enforced using bus lane contraventions but obviously the law disagrees ! Apologies to all.
The reason councils do this outside London is that they can enforce bus lanes under bus lane legislation, but until very recently, the powers in the Transport Management Act 2004 to enforce the sign on the basis of entering a street restricted to certain vehicles, had not been released by the government. In London a different Act allows such enforcement of Flying Motorbike signs used for LTNs and such-like. My memory recalls a very similar case in Nottingham, where the council issued bus lane PCNs for a Flying Motorbike sign. This one caused a huge row, because there were no bus services along the street !