1) Because it's not an Amazon problem, it's a fulfillment center/warehouse employee problem that applies to just about every company in the industry I'm aware of.
2) Because Amazon is a for-profit company, not a charity, and its responsibility is not to pay employees more than it has to or give them more benefits/easier working conditions than are necessary. Are you also demonizing your own employer for not paying everyone as much as it possibly can without going bankrupt?
3) Because in many areas where Amazon warehouses operate, they are a significantly better job prospect than other options, working conditions be damned.
4) Because competitive markets don't let companies pay their workers whatever they want to. If you pay your workers too much, a competitor will pay theirs less, pass the savings on to customers, and take market share from you. Amazon is one of the wealthiest companies in the world because it doesn't pay all 600,000+ of its warehouse workers an amazing salary and let them work in better conditions.
A lot of people who are angry at Amazon are really angry at the outcome of unchecked capitalism. These are the frustrations that should be applied to voting, not to perfectly legal corporate policies.