Amazon isn’t a monopoly the way that AT&T or Comcast are. People have a choice and can easily go to Walmart, Target, or most any other retailers online or at a physical location. In this case, Amazon is winning by out-competing their competition. That’s not monopolistic, it’s winning at competition. Fairly, I might add.
One way they’ve grown is by allowing small companies to easily add their products to their stores. Good luck getting Macy’s to sell your custom line of trollface shirts. Shelf space is live or die for small brands. Amazon has a lower barrier to entry without the strong arm tactics big retailers have used for years.
Retail is often a middleman operation with exclusivity deals and price fixing. Brick and mortar is failing for so many reasons. It’s not that there are restrictions preventing fair competition, it’s that the old guard is slow and incompetent and likely won’t survive the old model of exclusivity on goods.