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But Amazon isn't a monopoly is any way.

The WP has covered various union's arguments against Amazon, eg: https://www.washingtonpost.com/news/wonk/wp/2017/07/17/why-t...

I think you may misunderstand what left-wing people generally think. This quote from the above article summarizes it quite well:

Jon Leibowitz, a former Democratic chairman of the Federal Trade Commission, said the agency has a duty to challenge any deals that could end up hurting shoppers.

“Think about the Amazon business model — that’s typically bringing down prices and enhancing innovation, which is a benefit to consumers,” said Leibowitz, now a partner at the law firm Davis Polk. “But the FTC is a very professional agency, and they’re going to pay attention to any substantive concerns raised by outside parties.”