I disagree! You see this argument repeated around a lot: “well, the employee accepted this job, so since it's their choice, there's no injustice, they can always decide to go somewhere else!”.
I think this is missing the point. Do you think employers coercing employees into accepting conditions below what they should normally get is not a thing that happens? The employer, as opposed to the employee, has the power to manipulate the socioeconomic circumstances around the employee. They can (and do) prevent employees from discussing politics at the workplace or unionising, they can (and do) hide (and manipulate) information about who is getting paid what, they can (and do) decline additional compensation when it should be given (overtime), and this is the least of it.
You might think, “well, the employees should just get a different job then!”. But don't we want to fix the underlying problem instead of just churning our way through countless jobs? Sometimes it doesn't even work — you are often compelled to stay at a job for various reasons (you relocated with your family, you've worked there for a long time etc.), and you might not even realise this manipulation is happening to begin with.
The above comment said that employees will go very far to justify a raise. Don't you think that, if employees had all of the information their employers had, a lot of employees _would be_ able to justify a raise that they deserve, perhaps even without having previously realised this because of the information asymmetry? That sounds like an injustice to me.