I think the simple answer here is that the box (which I'm assuming was a Priority Mail box) has PRIORITY MAIL printed on the side to help workers locate expedited packages.
I might also speculate that as boxes are included in the price of a Priority or Express shipment, they would want you to pay the higher rate as they don't really know if it's a recycled box or not.
Generally speaking, I've always been satisfied with the USPS. Priority Mail is a pretty good deal for smaller packages when you consider the options. Most Post Offices are conveniently located as well...
Although, just the other day I wish the freaking mailman could have waited a few more minutes for me to come to the door so I didn't have to chase him down to get my new bag
-spence