People have choices. They can work for another company, they can start their own business, they can go live in the forest and live off the land. People can do whatever they want.

If they want to get money and work at a specific company, don't claim they are slaves. Amazon makes an offer: hey, if you work here you get X amount, and they accept.

You can claim an employee is a tool for the company, but you can also claim a company is a money generator for an employee. It's a work-money trade between 2 parties. Don't claim it's slavery.