You left out the "and they can freely leave at any time" part. Your analogy doesn't make sense once you add it back in.
So? That doesn't excuse anything either. If my plane crashes next Jeff Bezos's Antarctic base, and he lets me in on the condition that I submit to daily sadistic beatings for his own pleasure. Does the idea that I could leave at any time (and freeze to death) make his beatings OK? Of course it doesn't.
Assuming Base Bezos is the only one around, without it you would have had one choice, death. Now you have an additional choice, survival with daily beatings. He has made you unequivocally better off, despite being a sadistic evil billionaire. Capitalism can be weird that way.
Now - in a scenario closer to the reality of an Amazon warehouse worker - let's assume there are hundreds of other bases around, each with their own terms, with little barriers to entry keeping other bases from being built, and Base Bezos is the one with the best terms. Then again, it's existence makes you better off. If it didn't exist, your choices would be worse.
All the workers at those warehouse jobs (a job far better in every way than the factory jobs I had) are better off because those jobs exist. And people at comparable jobs are better off too, because their employers now have to compete with someone providing what is in reality good pay, benefits, and working conditions for entry level manual labor jobs.
You should note that's a straw man, since I did not make a comparison, rather I used a different example to illustrate a concept. To spell it out in the simplest way I can think of: voluntarily agreement to a contract does not make the terms of that contract right. That's true for all wrongs, including slavery and bad working conditions.
Then why should I bother paying the mortgage to my house or car because that APR sure ain't right in them terms. Dam' bank found me desperate for a new BMW and is slaving my balls off!
You're being obtuse.
And no, I'm not comparing you to an angle.
Do you realize that doesn't excuse anything? A rather extreme example to make the point: slave-holding is still wrong even if you can find someone desperate enough to voluntarily sell themselves into it to you.