I have some karma to burn, so let's do this again. Underpay compared to what?
There has to be some comparison here. You can't say "underpay" and leave it at that.
You might try and weasel and say "underpay compared to a decent living wage". Well, I have news for you - even that's relative.
A fair comparison would be "underpay compared to the rest of the warehouse industry". Which begs the question - why do people go work for Amazon, then? Are they trapped by not being able to leave the small towns where Amazon hires? Doesn't sound likely. Could it be that their overall conditions are more or less on par, or quite probably somewhat better than the rest of the industry?
I'm going to call your bullshit and say what I think directly: Amazon is a cheap target for young idealists simply because it's visible, it's fashionable to attack, makes a lot of money overall, and most people attacking it have no idea what's a good pay for that kind of work because they never did that kind of work.