This behavior is not limited to Amazon. Big-box retailers and department stores also ban people who abuse the return policy. And, as an honest shopper who likes being able to return things from time to time, I'm glad for it.
I think the troubling issue for some is that they can return things for perfectly good reasons, the items in question were in fact faulty, or mis-described on the website or whatever, but Amazon canceled their accounts regardless.
Of course it's possible that those who go online to complain about this are in fact abusing the system, but from Amazon's POV it makes perfect sense: they don't want picky customers who will return borderline goods because it will eat into their margins. It's much more profitable to sell to those who won't notice the dead pixels on their laptop display (or whatever the problem happens to be).
