Amazon employs about one million people in the United States, and unionization and collective bargaining could easily result in a doubling of their labor costs. This would certainly benefit the local economies surrounding Amazon warehouse locations, but would mean profits and dividends would fall. This is a significant number, an increase in labor costs by about $30B per year.
Amazon's gross profits vary year-to-year, but are on the order of $150B, so they can clearly afford to pay their workers a wage that would allow them to move up to respectable middle-class status - and this would also likely result in less employee turnover as well. The investors and executives would have to absorb the costs, but isn't that reasonable?