PBS Newshour had a good segment on all this for their Thursday night broadcast. The logic is taking advantage of human psychology - you're given a certain portion, and once you're done with that, that's it, you're done. However, it's definitely flawed, as nothing stops people from buying multiple drinks or just getting them at grocery stores and convenience stories, which are exempted from the law. I can understand trying to do something given the huge problem of obesity, but I'm not sure this will work. That said, education on the matter alone, which is what one of the people who was on PBS kept advocating, isn't doing much either to stop people from overindulging in sugary drinks. Anyway, for those interested, here's the link to the PBS segment.
http://www.pbs.org/newshour/bb/health/j ... 05-31.html