The news story is careful to note that the feet weren't deliberately severed, but they floated to the surface after natural decomposition eroded the joint. The shoe is buoyant enough to float the body part to shore. (and there may be many other such feet out there that no one bothered to look inside for)
Which I guess should be reassuring, except I now must presume that the rest of the bodies have been weighted down enough to stay put. Puget Sound has become a dump for murder victims just like the storied East River.