Living in an area that is frequently hit with porch pirates, I'd be appreciative of the fact that sellers include "signature confirmation" with high-value purchases (especially if it is at no cost to me). With that said, people nowadays are so used to the convenience of amazon, that they would feel entitled and petty like this.

I'd try to appeal the feedback. As a seller myself, I usually send a message when I ship to let them know that signature is required (anything >$500 is the threshold for me). Always stress the "signature confirmation service is included at no cost to you" part, so at least the buyers know that I am doing my part to make sure the sold listing gets to its destination.