Heard rumblings there might be action taken sometime soon:

We know everyone does it: they take Amazon GCs as payment for services (taxable income), then launder the money by purchasing stuff on Amazon (helps Amazon), then unloading it on other sites (helps Amazon by lowballing brick-and-mortar competition).

With the staggering amount of money the feds could make doing this, I'm surprised it hasn't happened yet, but that seems to be about to change.