> why shouldn't Amazon be allowed

Antitrust law used to address this. Monopolies are not allowed to do things that non-monopolies are allowed to do, because the effects of monopolies doing them are different than the effects of non-monopolies doing it.

Of course, Bork rewrote monopoly policy in a way that opens the door to exactly the sort of monopolies we now have. But it isn't as if this is some new notion invented because people hate Amazon; it used to be the rule.