You aren’t arresting them for drug addiction, you’re arresting them for doing drugs on the subway. It’s a quality of life issue. If you make it very clear to people that smoking crack on public transit will get you arrested people will stop doing it. This is from someone who thinks all drugs should be legal, but even if they were all legal they should be illegal on the train.Kick them out, give them a ticket. If they are a repeat offender, escalate at that point. I'm not sure how arresting them for their drug addiction is going to improve things if you're not going to be able to hold them. But I'm sure if they want to do drugs, they might choose to do them somewhere where they won't be hassled, and that might be a little different of a situation compared to your homeless encampment example.
Yes, that particular activity is a waste because we don't give people without homes housing - it's instead treating some symptom in one of the worst ways possible. They're going to go somewhere. They probably shouldn't be clearing it unless there is actually a place to give those people, like some sort of supportive SRO-style housing.
It’s just like open container or public intoxication laws - drink all you want in a bar or at home but you should be ticketed or arrested for getting shithammered at a playground or whatever because it hurts everyone else’s quality of life.