It's self-anecdotal i know, but since Ive been in london (nearly 9 years) I can't think of a single incident of that kind (people on the tube drinking and being antisocial). I'm not saying it hasnt happened, but it wasnt quite the plague on your house that it was presented as.
The reason the change came in was simple. It was something Boris could be seen to do immediately (since he's in control of TFL and can make byelaws) that would play well in the papers & the rest of the country. It could have been anything, they just happened to choose this.