By an international treaty to which Syria is part. They were banned because they were normally used to in conjunction with Chemical weapons. It is of course perfectly legal to prepare for war of self-defense, but in passing this technology to Hezballah (which I highly doubt happened), Syria puts itself at risk of becoming a target of international action against it.
Man shot dead in northern Israel town of Bi'ina, police say the murder was gang related (Haaretz)
from the article: Satellite images reveal: Hezbollah training in Syria missile base