Upvoted that one though - this begs the question:

should you construct an argument only by its premise, or should you try to reach the person? I think [s]he assumed that you are not a prison abolitionist and tried to find an argument that might sway you.

Is it better or worse to argue like that?

There was no prior mention of the person being a prison abolitionist. There was mention of concern about private prisons. I don't know how I am to intuit what a person is truly concerned about vs. what they state they are concerned about.