Yeah you can device such a reasoning - theologians from monotheistic religions do that. I guess for some this is comforting. It is certainly a bleak world where anybody can be hit by such a tragedy without any higher reason or justice. I just happen to find your viewpoint ("really, its for their own good") much more bleak and horrible.
I know that I'm stepping on thin ice here, but: My son does not want his teeth brushed. He absolutely hates it, because he does not yet understand that this pain is way lesser than the pain and treatment of tooth decay. So, I still have to do this to him, at least twice every day, because it is necessary.
I DONT mean to say that bone cancer might be good for something eventually, but; if it were, then we might not understand it, because we don't have neither control nor insight over the future.