If you want to work in my work place you have to be drug free. its a simple requirement. so why do so many?

There is an economic concept here called Supply and Demand. There as always been a demand for drugs and there are always criminals who are going to supply that demand. Citizens have historically funded law enforcement activities to just barely keep the situation contained. In order for more drugs to get off the streets, citizens are going to have to give up their civil liberties and live in a police state.

cough syrup is a over the counter medication that can be purchased at a pharmacy legally testing positive for cough syrup isnt against the law. and cops are just there for the allusion of saftey

Cough syrup may be legal, but abusing it isn't. It is just as illegal to chug 3 or 4 bottles of Nyquil as it is to over drink. BTW Nyquil is 10% alcohol, and used to be 25%.

Because people are idiots and more often than not the cops are busy with more obvious crimes than one moron who's hopped up on who knows what.