Any law is made for the betterment of society after long discussions surveys researches and it takes long long time before implementing it. On the other hand law breakers are very fast in breaking all the barriers find loopholes to break the system and suceed in getting gains from each and every circumstances. There may be rareest of rare foolproof system or law which could not be broken by cheaters. Ultimate truth is cheaters will gain whatever weapons you make for security and innocent people are always suffer losses like happening in merit system here.
This is why research chemicals can be legally sold, because the chemical formula is slightly different than the illegal drug. A law needs to be passed before the research chem is made illegal and this usually takes a while.