Stanley Fish argues that plagiarism is not a “big moral deal” because the taboo against passing off someone else's work as your own is just an arbitrary disciplinary convention.
Fish asserts that “the rule that you not use words that were first uttered or written by another without due attribution is less like the rule against stealing, which is at least culturally universal, than it is like the rules of golf.”
Let's concede this point for the sake of argument. The rules of golf are morally neutral. There's nothing inherently virtuous about playing the ball where it lies, that's just what the rule-makers decided would make for the best game. Many of the rules of golf could be rewritten with no moral consequences. There's nothing morally special about 18 holes vs. 19 holes.
However, even within golf, some rule changes would be morally loaded. You couldn't add a morally neutral human sacrifice rule. Rule changes that unfairly disadvantaged certain players would also be a moral issue. The controversy might not get much play outside the golfing world, but it would still be moral principles at stake.
Once you accept a set of rules for golf and start playing with other people who agree to those rules, deliberately breaking the rules to gain an advantage is cheating. Like stealing, cheating is universally frowned upon.