you're not very good at this thinking thing, are you?
here, i'll explain it in simple terms for you:
because cocaine use was perceived (rightly or wrongly) as being mostly used by blacks, marijuana by mexicans, opium by the chinese. the original banning of cocaine, for example, was "justified" with lots of propaganda about cocaine-fueled blacks raping white women.
it demonises the users (and their entire race or subculture - "dirty hippies") and gives the cops an excuse to arrest them and the courts an excuse to convict and sentence them....and for the last few decades with for-profit privatised prisons it's a way to legally enslave them.
(it's not just corrupt cops and politicians and high-level drug dealers who are against legalisation of drugs, the private prison lobby is dead set against it because legalising drugs would greatly reduce the number of potential slaves)
it's not the USE of the drugs that suppresses people, it's the fact that certain drugs associated with particular subcultures were made illegal - the same way, for example, that banning rap or hiphop music (while laudable in itself) would disproportionately affect black people. or, for a real world example, the refusal to play black music like jazz on radio until whites like presley started appropriating it in the 1950s.