It's far easier and cheaper for them to ignore most cases and instead wait for the few that blow up like this one and then look into them at that point. In those cases if they think that they're even remotely in the wrong I suspect that they'll reinstate the account, send their apologies, send the person in question a gift voucher and get on with doing exactly what they were doing before.

It's very easy to think that the sort of outcry that happens on HN, or even on Twitter, reflects public opinion, but the reality is that this audience is both far more aware of these events, and far more considered in their reaction. The sad reality is that Amazon can get away with this sort of thing because most Kindle users won't ever learn about it and even if they do won't really understand what it means or change their behaviour.