Drug dealers aren't jailed because they're "ruining their lives," they're jailed because they ruin the lives of others. Addicts are a drain on resources that generally contribute far below what is required of them to maintain society.
Contribute or perish!
Drug users are arrested because they are not contributing and there is no way person could just be a person and not a nice productive bee
Do you think imprisoning somebody because they have an addiction messing with their life is too easy? Or is that just for the dealers? The previous post mentioned drug users.
Which highlights that making drug use illegal is far more about "You using drugs is unpleasant Tobe near, so I'm going to send you to jail" than anything else.
Making drug sales illegal unless through mandated sources make a lot of sense because of the harm it can bring.