the fins are chosen due to space available i would suggest in the B2 bomb bay. the bonus of them is they have a very low aspect ratio for lift generated and with the momentum of a 30,000lb device, you need a lot of lift to change the flight path. as for accuracy, you need to pu these things inside the bunker, or as close as is possible .
similar fins on russian AAMs. poor manoeverability through the transonic region apparently. all on Wikipedia.