They are certainly enforcing the laws against selling and purchasing drugs. It is uncharitable but if the state actually cared about people it would treat drug addiction as a health issue. It can be treated, current policy obviously has done nothing to solve the problem in the US. The police are enforcing the supremacy of the state, these people are flaunting the law and must be punished otherwise the government looks weak. Nobody wants to admit defeat because it is admitting they are much less powerful than they want people to believe they are.

All of this is conjecture and would of course be unconscious motivation. People want to think they're the good guy, people justify what they're doing so they can sleep at night.