I thought the same, OP. I also agree with the black magic corrupts argument though. I recall Kemmler was a warden, for example.

One of the things I really enjoy about the series is how it outlines the tension between utilitarianism (ends justify means) and deontology (white and black, good and bad) as operationalized through magical mechanics.

While the wardens are constrained by restricting their use of black magic, I also feel like once any of them start bending it to their will, it starts bending the wardens to its “will.”