Well, it "should" have been an arm as well as the stump you actually get. Bottom line is, when you see the stump in the movie you also see the arm, so not sure on the logic of not including it.
Well you have the regular arm. So you can create that memorable scene where he has the shredded arm, and then finds a perfectly new arm to replace the one the Sand people ran off with.
Logically it's cheaper for SS.