Leaving that amount of money anywhere other than in an FDIC insured bank is incredibly irresponsible. Arguably as irresponsible as Apple's decision to require the last 4-digits of a credit card to gain access to iCloud, where a malicious intruder could arguably do way more damage, on average.
I could walk up to an ATM behind someone and get the last 4 of their card all day long. It is printed on every single receipt you've ever gotten.
In addition to Amazon actually being an FDIC insured way to store your money, it should be realized that your bank is only going to have you covered up to $250k. Do you feel like storing more than that much money for a company is then "incredibly irresponsible"?
Yeah, I do actually. Why would you need more than $250,000 liquid currency in the same bank? Nobody does that.
Anyway, you are wrong about Amazon being FDIC insured like normal banks. They can potentially insure you up to $100,000 via pooled accounts stored at FDIC banks. Amazon payment accounts themselves are not FDIC insured. So you can imagine how much fun it would be to get your money back as compared with a real bank.
You are not "potentially" insured up to $100,000: you "are" insured up to $100,000. This is critical to note, as the disdain you expressed was for people storing, specifically, "upwards of $100,000 in their accounts on Amazon".
As for storing $250,000 in the same bank, I am highly confused what a company otherwise does. I have a company, and while I don't actually own a lot of the money that I hold on to (it is almost entirely held liabilities for things like sales tax or vendors), at any given moment I am certainly holding more than $250,000.
Do you then contend that I should be having numerous bank accounts to hold this money? I can't invest it, as I need to have the money to pay the aforementioned liabilities at the end of pay periods that are too short to move money in and out of investments. (Note: I also do not believe my business is somehow crazy-weird.)
I could walk up to an ATM behind someone and get the last 4 of their card all day long. It is printed on every single receipt you've ever gotten.