You can be prosecuted for riding a bike without lights after dark; you can be taken to court if you knock down someone who is riding a bike without lights, and spend taxpayers' money trying to prove your case. There's the cost to the NHS treating accident victims. Cyclists are safer when they have good lights – I have noticed that on a wet winter's night an unlit cyclist literally disappears amid the glare of car headlights and wet reflections – they don't realise how invisible they are.
We are trying to encourage greener methods of transport and better health: cycling ticks both boxes, but only if cyclists don't get knocked down! Make incorporated front and rear lights mandatory on cycles, as they are on motor bikes etc.