The problem I have with the USPS on this issue is that you caught grief over something you didn't do. They'd do better to send a nasty-gram to the shipper than to talk to you. I'm also sort of surprised that they didn't open/inspect the package and tape a wad of paper over the knife tip; still, it's good that you got your knife and that nobody was hurt.

BTW: how hard does one have to fling a package to get a Muk blade to slice through the box like that?