Because it is specifically forbidden by the USPS. They do print something on the inside now so you can't turn them inside out and use them as free boxes for other services, but in the past, they didn't. I guess they encountered exactly what I did--that sellers on ebay were getting priority mail boxes and cutting them up for packing material. I reuse them when I get them, but not on the inside of the box. I reuse them as priority mail boxes.