[junto]

I'm guessing that Amazon suspect that the user is doing something that they feel is deliberately untoward, but that they cannot prove.

Therefore, rather than feel that they are being cheated, they decline to have the person as a customer any longer. Whilst I find it hard on the customer if they have unused gift cards on their account, I have no problem with Amazon choosing not to deal with a customer that deliberately is trying to exploit the company.

It is no different to a physical shop declining the custom of any walk-in customer. That is their right, as it is our right to spend our money there or not.