Exactly how does one draw the line between punishment and treatment? Our society has blurred the two notions.

Very true. As I noted in my other comment, jails necessarily suck to be in, above and beyond the loss of freedom of movement.

We just don't have a common, accepted protocol to handle people who are "dangerous to others, though they haven't (yet) done anything wrong, and maybe did good by turning themselves in". Such people would deserve to be detained, but not in a way intended to be unpleasant.

The closest examples I can think of for this kind of treatment (other than the international border system I described in the other comment) are halfway houses, quarantining, jury sequestration, and insane asylums (in those cases where the inmate has just gone nuts but not committed violent crimes yet). There needs to be a more standard protocol for these intermediate cases, which would look similar to minimum security prisons, but not require you to have committed a crime, and be focused on making you less dangerous so you can be released.