The malt syrup in the book is a symbol for the extreme poverty and hunger felt by the citizens of Dresden. Despite propaganda, there is not enough food, and what food exists is not very nourishing. The syrup, made for pregnant women, is "enriched with vitamins and minerals" so that the pregnant women can give birth to healthy babies. Naturally, the workers who make it steal spoonfuls of the syrup when they can; the act is called "spooning."
The syrup tasted like thin honey laced with hickory smoke, and everybody who worked in the factory secretly spooned it all day long. They weren't pregnant, but they needed vitamins and minerals, too. Billy didn't spoon syrup on his first day at work, but lots of other Americans did.
(Vonnegut, Slaughterhouse-Five, Google Books)
Spooning is illegal for the obvious reason that it is stealing. Less obvious is the matter of control: the workers are intended by their masters to remain weak and hungry; this allows the workers to be more easily controlled, and keeps them from having thoughts of uprising or rebellion. Spooning syrup is a minor act of defiance; yet, it comes out later that there aren't enough nutrients in the syrup to be truly useful. This indicates that while spooning is punished, the masters of the factory are well-aware that everyone is doing it, and so are not worried that it will foster rebellion. After all, the act itself satisfies the urge to rebel, and the lack of real nutrition means that the workers will never be strong enough to mount a revolution.