I can see a fairly intuitive argument for calling the practice of law zero-sum ("gain for one side is nothing but an equal loss to the other"). It isn't right, but I can see how people would come up with it. You might have something else in mind; do you think that the lawyers working for the ACLU are engaged in a zero-sum activity?
I don't understand applying the term "zero-sum" to management at all.
A craftsman takes wood and creates a bench. The bench is worth more than the wood. Thus, he created value.
A manager does not do anything like that. He does indeed do valuable work, by enabling others to create value, but he does not create value himself.
But you are correct, zero-sum is a bad name for this. Do you know a better name? I was thinking about stock trading, where the money one wins, another loses, thus zero-sum.
I don't understand applying the term "zero-sum" to management at all.