There are two lines.
First there is the matter of being ripped off. Someone makes widgets. They get paid $1 per widget. The widgets sell for $10000 each. The materials and such to make the widget don’t cost anywhere near $9999. The employee is being ripped off. They are trading very valuable labor for not enough dollars. Like if someone traded 100 sheep for 1 wood. Just unfair.
Then there is the cost to the employee to do the work. When anyone works it costs them time, effort, opportunity, health, etc. No job should cost the employee too much. Nobody should have to sacrifice their health for someone else’s profit. Also, regardless of how much the widgets are selling for, even if the employer is taking a loss, compensation must always cover the employees costs to work and live. Someone makes five widgets that sell for $1 each, but they still need to be paid a few thousand dollars because rent and food are a thing.
Even if both lines are crossed, it is still in the best interest of an employee at a for-profit enterprise to get as much compensation as possible in exchange for as little work as possible. At least trade 49 sheep for 51 wood to turn the tables a little bit.