It's voluntary on some level sure, but it's also true that in any single 'negotiation', Amazon has an enormous advantage by virtue of size. Corporations bargai

It's voluntary on some level sure, but it's also true that in any single 'negotiation', Amazon has an enormous advantage by virtue of size. Corporations bargain collectively automatically, because they're already collectives. This is especially true in the case of enormous corporations with many thousands of employees. When someone is getting hired by Amazon, it's literally a million versus one. That's a lot of leverage to bring to a negotiation, so it makes sense that a union would be necessary to even the odds for unskilled labor.