That's fair, and unquestionably a conflict of interest. However, it's not an irrevocable quality of a prison run by a private company (and therefore evil "by definition").

OTOH, purchase of shares is at least an above-board, traceable interaction. Government officials have always gotten perks and kickbacks from private companies, rich individuals, NGOs, and their own budgets that are either off the books or disguised as something else, and therefore less transparent.

I haven't seen (or yet hunted for) a study of publicly run vs. privately run prisons, in terms of treatment, costs, etc.; but I'd be curious, to say the least.