These people could be just not working at all, in which case they would be more of a tax burden.

What policy rules would you put in place that would stop "the taxpayer is subsidizing Amazon" while not hurting these employees?

Alternatively, it means that Amazon is able to underpay because workers are able to go on food stamps.

Amazon would be just fine if they were forced to do the right thing now and then. AWS alone nets $20+ mil a day in profit, there's no reason to evade taxes globally, call your workers contractors, have them piss in bottles or eat off food stamps. There's no reason to have a store full of counterfeit crap and fake reviews either when they could have appropriate due diligence instead.

These are all easy problems to resolve when you are the world's richest man and revenue's still growing.

They're also easy problems to ignore if you're OK with being awful until the EU forces you to meet a minimum standard of conduct.