(group) Why might a multinational corporation with identical plants in different countries pay different wage rates to workers in two different countries even though their skill levels are the same? Does this strike you as unjust? Why might the higher-paid workers object?
Multinationals might because there are fewer opportunities for workers in country B than country A. So there is a higher supply of workers at the lower price. If they had better opportunity they'd go for it.
It's unjust for the workers of country A. But extra-just for workers in country B. Humorous phrasing aside, someone in country B would consider it unjust that the money goes to someone in country A when they are willing to do it for less. Workers in country A would consider it unjust as they are unable to make a profitable living due to costs that are higher than what are those in country B make. Either way there is injustice.
Higher paid workers might object because the work can be moved to country B, and their opportunities are then fewer. So either they take less money from the multinational, or they take less because they are laid off with X fewer opportunities available, and they take less because there are fewer opportunities.