If you want to sell something online you can do it in any number of places, and you can do it yourself (and fairly painlessly). If you want to buy something online, you can go to any number of retailers. You can get an Echo from Best Buy if you want, and you can get towels delivered from Walmart etc etc.
Ditto cloud computing, of course -- it's an incredibly competitive market.
Amazon does have large market impact and can (in a practical sense) exert market pressure. That's obviously a dangerous position for everyone to be in, but it's far from monopoly and it's also (afaik) well within the bounds of (American) law. As I understand things, US antitrust laws are all about abuse of market position, not just the existence of a dominant position.
