[A CA Guy]

Because the druggies were breaking the law and attempting to grow their criminality base. Drying out in Jail is just a side benefit. :-)

[Know your rights]

Why is it government's business to force "worthless druggies" to "straighten out"?

Because the druggies were breaking the law

That doesn't answer the question, which is why the law prohibits drug use (except for the drug use it allows, like drinking).