Addiction potential is heavily tied to one's circumstances. The more desperate the circumstances, the greater the abuse potential. For example, you specifically listed crack as opposed to run of the mill powder cocaine. The only difference between them from an abuse potential perspective is the purchasing power of the user. Likewise, the raver who's popping some mystery mix of amphetamine, methamphetamine, and MDMA at a party once a month is far less likely to succumb to addiction than poor sap living in Redding where the only thing available for entertainment is smoking meth. Opiates are self-selecting in this regard: they're not that pleasant relative to other drugs, except for their ability to make you content, which only really appeals to the desperate.

If we follow your "it's illegal because there's a probability" theory of law enforcement, then it should only be illegal to do drugs while poor, which is pretty much where the law stands today without the deceptive veneer of equality. Now, that should sound like an absurd legal take, because it is. The reasonable response is that it's poverty that's the problem, and it's not a problem we solve by throwing all the poor people in jail.