Isn't it tautological to claim that legal definitions are not arbitrary because of legal spheres? As to ethics, people often invent ethics that coincidentally benefit themselves. I'm talking about economic definitions.

I thought a bit more about your ethics argument.

If you agree to provide X labor for Y $, and later say you are "ethically" entitled to Y*2, how is that ethical? Ethically, you should stick to what you agreed upon.

The ethics are about how you construct laws around employment based on the sorts of employment relationships the society considers just or ethical. For instance in many states there are regulations around workplace safety. Given the power differential between the employer and employed and general human stupidity it is understood that there are certain limitations that must be placed on an employer to protect the employed. This similar to how society recognizes certain obvious limitations to transactions such as purchasing food, medicine or the construction of Opera houses.

How does that fit in with a worker not keeping his word about what he is to be paid?