They're paid in victory, which costs nothing and which you can't eat. That is fundamental to operating something like Amazon, which is designed to avoid switching into the 'rake in money' stage of capitalism as long as possible, in order to compete more harshly with other organizations that DO try to rake in money as soon as they can.
Amazon understands that in the context of organizations it stands to benefit from being seen as the meanest competitor, and that it can get public investment at scale by making this case. The argument isn't that it will hand more money to investors directly, it's that it will attack other things one can invest in, and make those things lose, and therefore it's the safe bet.
In so doing, the adoption of a crazy, aggressive, Spartan attitude among all its workers and management is beneficial, because the easiest way to convince the world you're a psycho axe murderer of a capitalist is to actually be one. In other words, 'making massive profits' isn't the end goal: 'killing other companies' is. And so, the workers do need to be paid that little and treated that harshly because every worker must be first a warrior, willing and able to trade away their health and well-being for the betterment of the company, and this goes all the way to Bezos, whom I'm convinced is going to live a shorter life than a billionaire might otherwise expect, from stress related damage. It's that or go soft and see his company go soft, and I think if he could do that there wouldn't be an Amazon as we know it: people will not follow a leader who doesn't at least pretend to represent what he is leading people towards, and Bezos is leading Amazonians to be warriors and throw themselves into the wood-chipper in order to destroy all competition.
They're paid more than they would be at easier, more civilized jobs. Part of the compensation is this emotional compensation intimately tied up in the American attitude, the cost-less, intangible compensation of knowing you are on the winning team. That is inherent to what Amazon is.