You may not need them, but often enough you will have to pay the duty and VAT before the customs release your package, thus erasing the price difference. As for the warranty, customers usually do need it.

Well, I and many others did. The customs officers don't have to believe what's written on the package.

Those who pave streets you walk on and fine you when you're speeding... So uh... How about Aliexpress?

I think this is valid point in general, but I don't think it has much bearing on a discussion about Amazon, which doesn't contribute very much to taxes.

They pay VAT and custom duties as much as anybody else. They probably skim on employee-related and profit taxes though.