They do virtually nothing to police their marketplace. Meaning that it has become the ideal place for thieves to purvey stolen goods. Which incentivizes the thieves to steal even more. Which then has cut even further into the profits of their brick and mortar competitors facing ever more and ever more brazen shoplifters and other thieves. Quite the virtuous circle, for them.

[insert mandatory statement from Amazon spokesperson about how they take theft very seriously and work diligently to remove sellers who are abuse their terms of service.]