If it's worth doing, and in the case of Law necessary to a functioning society, then if you regard society as positive sum then things that are necessary for it to function are part of what makes it positive sum. You can't strip them away, they don't exist on their own, they are part of the system itself, they exist in a context, and discarding that context over simplifies to the point of the absurd. It's like saying sanitation is positive sum but one of things that is necessary to make it work, proper plumbing, is not positive sum. You can't strip off the necessary pieces of something like that and regard them as things in and of themselves.
Salary negotiation is an excellent example of your blinders in this. I suppose you see it as the employee gets X dollar, employer loses X dollars, so it must be zero-sum. That get's too hung up on the easily tangible piece of the transaction and ignores the more complex context in which the negotiations take place. In truth, both employee & employer get something out of it. Obviously the employee gets a better salary. The employer, by providing the employee with the things that help make them a satisfied worker, gets higher productivity than otherwise, and perhaps a higher guarantee that the employee will stick around. Both win. They are cooperating, literally part of what game theory, from where the entire concept of zero sum comes, regards as integral to many positive sum outcomes.