To echo a sibling comment: freedom, not salary, is under question. Agreement should not be given one time at sign up, it needs to be given continuously. High cost associated with being compelled to leave the country upon quitting, low mobility between employers, etc. reduce that freedom.

Worth reading about https://en.wikipedia.org/wiki/Forced_labour, it's not just the practice of chattel slavery, and various forms are still legal in the US.