Game-theoretic concerns. You don't want to collapse the hierarchy of deterrance, otherwise you'll have "might as well murder them to hide the evidence of my previous crimes" situation. Additionally the difference in visibility and number of people who are in a theoretical position to commit theft vs. those who are in a position to withhold wages might have an impact since it's probably easier to go after the latter after the fact while you have to rely more on deterrence for the former.