Those guys were not prisoners, I’m fairly sure those were souvenirs and not punishment.
Odd to think of them as souvenirs - I get the impression that “lucilling” somebody is Negan’s personal monopoly, not something anyone else is allowed or encouraged to do, and since Negan’s schtick seems primary to club one member of a group to improve the morale of the others, the Polaroids seem more like warnings than keepsakes.
On reflection, it seems like Negan maintains his power in part by exploiting hostages to fortune, i.e. if you attack me or one of my lieutenants or try to run away, I’ll kill one of your friends or relatives.
The problem with such an approach is that sooner or later he’ll run into a loner who has no such hostages, and thus won’t hesitate to kill Negan and everyone in Negan’s inner circle.
And Daryl implied as much when Dwight said (I don’t remember the actual dialog) that he kneels because it keeps someone else safe, and Daryl says that’s why he won’t.
Except Daryl is fooling himself. Rick, Michonne, Carol - he has loved ones and family.
The only thing that takes out a bad sociopath with a bat is a good sociopath with a gun.
In his cell of self pity, he probably thinks those people don’t care what happens to him now. I’m hoping for a Carol/Daryl killing spree.
At best, they’ll hint something like that is about to begin when they take their mid-season break.
Sorry. I was eating a Milky Way.