It's really bad. We probably want to not have monopolies like Amazon moving forward. So much so that we could probably pass laws to prevent it. Just in case we forget.
Overall the US seems to be incredibly open to monopsonies, monopolies, and oligopolies, and very hostile to any restraint on such things. Though, the merest hint of success from a non-US entity seems to invoke bipartisan terror and pleadings for sanctions and new regulations. Funny that.