Then you aren't their target market. They don't make their money off of people that are high-usage customers. They make it off of people who buy it and use lower then the predicted average.

You are assuming that their Business model is to profit from the difference in cost of shipping and prime subscription revenue. Rather than profit from a higher purchase volume if one has prime shipping than not.

I for one have observed that I have bought more things since I have had prime shipping. There is something to be said for the quick response to your order in the form of 2 day delivery.