I don't really want to get into the Amazon return shipping ethics, but just for the information of the thread, Amazon has two categories of returns:
1. Errors in ordering/shipping, defective products, bad product description
2. Wrong size/color, not what you wanted, changed your mind, not needed/wanted
Returns for category 1 are almost always free, there are some exceptions for very large, heavy, fragile, awkward, or items that are very expensive to ship for some reason but this is typically covered by shipping insurance
Returns for category 2 depend. Certain items are specifically selected by Amazon to allow you to "buy and try", most of these are clothing items, but there may be some others. Amazon notes on the product page that the product can be returned for free. For all other items, Amazon will accept the return, but will subtract the cost of shipping from your refund (the schedule for return shipping fees is available
here). Amazon will also subtract a percentage of the item's price for certain opened items as well.
I have spoken with Amazon customer service before with a similar question, and I was told that they want to allow users to return non-defective items, and that the intention of the return shipping cost is to allow just that.
While I agree that it is unethical to lie and say that the item is defective just because you want a full refund, I don't feel that it is unethical to be honest with the customer service representative about your reason for returning and accepting a partial refund and paying the return shipping cost.