Since the parable is about the kingdom of heaven and an effort by the landowner to get a harvest in maybe we should consider that there is a lesson here about God using the different people to reveal his plan in history like the Jews and and then the Gentiles are used at the end to help gather the harvest and they all will be rewarded the same if they have the right attitude and heart and are focused on getting the Owner's harvest in which after all is the main objective of the Owner.
He starts out with an objective to get a harvest out of the field and into the barn and sees that more will be needed to accomplish this goal. Then even more. At the end when the mission is accomplished the first laborers should be rejoicing that the latter laborers helped them accomplish the goal which would not have been done without them. Instead they complained because they did not have the master's objective in mind.
It is another parable exposing the wrong heart of the Pharisees and how they would be angry at the idea of the Gentiles entering into the promises of the covenant without even needing circumcision or be required to observed all the many Jewish laws and ordinances. It made them very angry and they cried "unfair."
Now he does use the concept that they agreed to a wage and had no right to worry about what another laborer agreed to. However this was also a daily job wage negotiation and the ethics of this wage negotiation can only be discussed in the context in which it applied which was for that day. The next day would have been a new negotiation.
But wage negotiations is not the main point. Getting the harvest in, and being rewarded for that is the point. Who has the Owner's objective in mind? On a side note an employee who thinks like the owner and is good at accomplishing the owners' objectives will get raises without even having to ask for them.