Selling zero day exploits is an amoral act.
That being said, your analogy is broken and doesn't fit here at all. First, parking lots are passive and have no intelligence. Amazon is neither of those things. Also, there are no such people who dedicate their career to lawfully testing the security of parking lots in exchange for money.
In the real world of mega corporation technology (which is almost as different from a parking lot as you can get) there are countless black hats motivated by money to steal from Amazon, and countless grey hats who would help combat or mitigate the issues if properly compensated. White hats are the rare exception and report issues even without compensation.
There are numerous grey hats who:
1) If there is a bug bounty program, would report the issue to be fixed
2) else, would ignore the issue entirely.
You know this is true because you experienced it yourself. The difference is you can't afford the service of a grey hat.
So forget the morality of these hackers.
Amazon refusing to pay out a bug bounty program is amoral. Because what you're seeing is Amazon trading the security of their customers, and for what? Greed and hubris it seems.