Paperwork and accounting -- it's way easier to just allocate a general slush fund of Amazon GCs or pre-loaded VISA cards than it is to get everyone to fill 1099s, pay accountants and what not just to pay people a few hundred dollars.

Tax avoidance.

Always a red flag.

Doesnt Amazon Prime credit have to be reported as cash for tax purposes?

That is if you get over $600(the IRS rule was $600 in late 90s not sure now) in Prime credit it has to be reported.