What you, and many others, fail to acknowledge is that employment is at will for employee and employer. Amazon is not chaining people to their desks. They are in a competitive market for talent and they pay market rate.
If you, as an Amazon employee, don't like the working conditions, pay, etc. simply quit. Go work for the better employer. That's true from an entry level job all the way to the C suite.
All of these arguments re: Amazon, WalMart, Nike, [insert 'Big' company here], etc make zero sense to me given indentured servitude was outlawed from this country about 150 years ago.
The other argument, re: monopoly, at least has a scintilla of reason to it. But, then again, competitors are just a Google search and a click away. I choose to buy from Amazon because it's easy and their selection, service and prices are excellent. If/when that no longer becomes the case, I'll shop elsewhere.
Case in point, I wanted to buy a Google Home product. Searched on Amazon. They didn't carry it. Clearly, they don't want to distribute anything that competes with their Echo, etc. products. So be it. I bought my Google Home and Minis from Google directly. No problem.
The argument that unionizing will open the books and uncover where wasted money is going makes no sense either. Are you saying that unions are going to somehow manage Amazon better than Amazon execs? If so, I beg to differ. Amazon is one of the best managed tech companies in existence.
Note: I don't work for Amazon, but I buy stuff from them all the time.