It's not reasonable or effective to delve into the private supply chain, IMO. Don't let perfect be the enemy of better.

You're choosing to avoid Amazon on moral principles.

It's kinda relevant that these principles only apply to services that have the Amazon logo, and not where more than half of Amazon's profit is coming from.

Hypothetically you might be bringing more business to Amazon through any of their AWS customers than by buying your USB cables on the storefront.

PS: I don't have a good answer to this, but boycott and "vote with your wallet" kind of actions have became a very complex thing IMHO.