I think they should've have fined him as that's what their law prescribes, but I suspect mummy and daddy may have moaned about that as well. In all scenarios someone would've bitched, so I guess they were thinking they might as well have some fun and teach Johnny a lesson.
When I joined the dibble I caught the tail end of the real old school, last of the pre-PACE coppers, and I've seen them do a lot worse to a Johnny Spottiebum with no lights, such as slash their tyres and other unpleasantness, and in Oz they don't have PACE or an equivalent ( in Oz if you're nicked you better go guilty or you'll fall down the stairs) and as a consequence they have a very high convcition rate cos people are quick to cough to any minor offence as quickly as the can for their own health. In all, Johnny didn't get off too badly at all for Oz.